I thought I had this problem figured out, but apparently I don't since the car refuses to run correctly. Here's the deal...

Car will start up fine, will idle fine other than RPMs jumping around just a touch. Give it light throttle, it stumbles badly...if I let off, it will either correct itself or stall out. It happens most often in heavy load conditions, such as starting moving, going up hills, or driving on the highway at low RPMs with the converter locked up. If I manage to get it above 2000-2500 RPMs, the problem seems to go away (possibly since it's not under heavy load anymore). It happens more often when the car is cold...if I drive it around for about an hour, it's not as bad, but it will still act up.

I knew I had a leaking header gasket, so I spent Saturday replacing the header gaskets, spark plugs, the IAC (since the one in there was going bad), and I also switched to a lighter weight oil since it's been awfully cold around here. None of the above fixed the problem, but it's not as bad (maybe a 10% improvement, but not enough to be worthwhile).

I'm thinking it may be a fuel problem and not an opti problem. Since it only happens under load, I don't think it could be the opti...unless something could cause the opti to only misfire under load (seems fine once I get it up to higher RPMs). Other than replacing the fuel filter and testing the fuel pressure, anyone have any ideas as to what else I can check? I'd like to get this fixed since stalling out every 30 seconds on the way to work is starting to suck.