1994 Mitsubishi 3000GT Engine misfire when warm

Last summer the air conditioning stopped blowing cold air. Then a few weeks later, it started misfiring slightly on slow accelerations. Then while idling after warming up. It does not miss at all when it is cold, if you floor it, or at cruising speeds on the freeway.

I took it to the dealer and they said it was the valve cover gaskets leaking into the spark plug wells. I have heard of this before on these engines. The check engine light is not coming on, but the computer is giving a random misfire code.

I did the work myself. I replaced the valve cover gaskets, spark plugs, and all three ignition coils. None of these things fixed the problem. I did not replace the plug wires as they seem fairly new and in good shape. I inspected the timing belt and it seems somewhat loose. (I could probably slip it of the cam gears without much effort.) I know there is a hydraulic tensioner in there that increases tension as the RPMs go up. The car now has over 160,000 miles, do you think I should replace the belt again?

Do you think the tensioner may be bad or leaking so when the oil warms up, the timing is sloppy? Could it be an EGR valve or an oxygen sensor. Could it be a vacuum leak? Maybe it is the air conditioner compressor going bad, causing the engine to be loaded down while idling, or slowly accelerating?

Any help you could give would be appreciated. I hate it when the Dealer charges expert rates, and can't find the problem. They just want to keep swapping parts until they stumble across the answer.

Anyone can do that, even me. And I don't charge $175.00 per hour. If I can't figure it out, I may be forced to take it to a Chrysler dealer. There are not many Mitsubishi Dealers around here. Many Chrysler cars have Mitsubishi engines.