I've noticed since the switch to winter gas i have a pretty rough idle, to the point where the whole car shakes a bit when stopped in gear. It isn't just the weather, there were a few warm days where the same symptoms existed, but the second I give it gas to go it runs smooth. Now I have my timing advanced to about 8* and have had no problems during the summer. Would bumping it down to 2 or 4 or even stock 0 smooth the idle out? I've already replaced a ton of sensors and fixed vacuum leaks due to a previous problem that has been taken care of. Anything else I should look at?

Edit: I also would like to stress that I have an l69 that uses the EFI system from my stock L03 so I'm not necessarily operating under normal conditions. This has been a problem diagnosing things before.

 

If it's shaking at idle, then smoothing out, check yoru plugs and plug wires, could be a cylinder misfiring. When was the last time you did a full tune up with plugs, wires, cap and rotor?

When my #8 plug wire melted to my headers due to poor routing I lost that cylinder. It would run smooth on the highway but as soon as I got down to idle it would get rough and shake a little.

 

Hmmmm. I did a thorough tune up lsat october when I swapped to my current setup. I did pull plugs 1-6 semi recently after I was running rich for a while and they looked and were gapped fine. I bumped the timing down to 4 degrees adv, and it did calm it down some but obviously I get poorer throttle response. Would like to find the real problem but I can deal with it for now.

 

It would be worth a check anyway. It's still running so the issue isn't that bad.