Every few seconds (Ranges from 10 easily to 100, feels random) my FPS jumps to 0 for less than a second and you can hear the engine sound stutter too.

It happens in both single and multi playing, it happens when I view replays, disconnecting the wheel doesn't help, turning the antivirus off doesn't help, I also defragmented, scanned for viruses and used anti-spyware applications but the PC was clean.

I'm running out of ideas. :/

Hmmm, sounds familiar... I had similar issue few months back.
When I turned my audio jack back and forth I could hear even more stutter. I found out my headphones wire or connector was damaged. No problems since I connected new headphones. Hope it helps as I don't think I can help you further.

Try reinstalling LFS

I tried using a fresh version of lfs because i thought all the high resolution textures might cause my ram to work faster than it can. Didn't solve the problem.

I'll try that headphones trick when i'm freed home from the army, hopefully on thursday.

It isn't a loose audio jack if your FPS drops.

This happens to me when a background process is working. NOD32 anti virus on my PC can update in the background and when it does I get about 5-10 seconds worth of stuttering.

If you've turned AV off then it must be something else. Try running MSCONFIG (not sure if this is in Vista but its a Godsend in XP) and trim down the unwanted stuff from the Startup tab. You'll be surprised at just how much crap is in there.

My pc has no crap in the background. Nothing that is not needed is in the startup. I also tried turning off all the working applications in the background, including the antivirus.

It must be something. Can you post your full PC specs?

Do you have a branded PC? They are usually filled to the gills with rubbish.

A 4 years old machine with 2x512 400mhz ram, a geforce 6800gt 256mb pci-e and an amd 3200+ @ 2ghz 939 pin venice core.

It was the top of the notch machine when i assembled it 4 years ago.

It runs stable at around 50 degrees (celcius)

What average frame-rate are you getting at the moment?

You will definately benefit from more RAM. A good 2Gb is the minimum these days. If you upgrade this is without doubt the way to go.

"...it happens when I view replays..."

I´ve almost the same specifications here.

Running a full grid replay and checking my work load (CTR-ALT-DEL):

Lfs.exe takes 210mb and I´ve a total of 700mb used.

The CPU work load is at about 10-20 % watching this full grid replay.

a) What´s your CPU work load watching a replay?

b) tell a little about your harddrive and your actual graphics driver and AA/AF settings as well.

My former pc had this problem as well, which had a 2800+ and a 6800gt. I think it had to do something with the sound lag option or the way lfs operates when a user leaves the pits.
I remember there being a lot of topics about this "sound stuttering lag" like 1 1/2 year ago, so maybe those topics have the solution for you.

edit: found a post about it: http://www.lfsforum.net/showthread.php?t=17268
It sums up the following possiblities:
- Sound hardware acceleration
- vsync implementation in lfs
- dual cpu problem, but im pretty sure your 3200 is a uniprocessor.

I'm still not home, so i still can't test anything. I'll check that url you posted soon.

The harddrive is a generic 200gb sata2 hdd running at 7200rpm. The game runs at 85fps stable because that's what my screen's refresh rate locks it at, due to using vertical synchronization.

I'll make sure to update my sound card drivers, as well as disabling vertical synchronization (i think i first noticed the problem when i installed the version that allowed using aa and af from ingame, that's when i stopped using nvidia's nhancer and used lfs's ingame settings to enable vertical sync).

Quote from BullHorn :

Yesterday 02:43...Every few seconds (Ranges from 10 easily to 100, feels random) my FPS jumps to 0 for less than a second..

Today 20:33...The game runs at 85fps stable because that's what my screen's refresh rate locks it at, due to using vertical synchronization.

Sorry, this doesn´t make sense for me.

10 to 100 seconds, not frames per second. What i meant is that the time between each stutter is absolutely random.

I'm still posting from the phone so it's often hard to type and make sense. I should af home in two days to try the suggested ideas.

Oh, and when i said it runs at 85 fps stable, i meant that the fps is always at 85 except for those 0.5s stutters that i have once every few random seconds.

I tried everything suggested here and it didn't help.

I'm beginning to suspect it might be caused by overheating. It's over 40 celcius in Israel these days, and my PC has no cooling devices, other than tiny fans. 50c when idle isn't too good, either. :o

What I always do in the summer is grabbing 2 80mm fans, use 1 for sucking in air into the case, and the other one sucking air out (the one sucking air out should be highest btw).
A fan near the window/doorway blowing some fresh air to your pc area works good as well.

btw, recently because of the heat I had this problem as well, and it turned out to be the northbridge chip (which was only passively cooled) which was constantly overheating.

Good luck fighting the heat