If we're making wild guesses I'd take a close look at the O2 sensor. They can start to get lazy and cause problems and not set a code. Ask your mechanic to monitor the cylce interval on the O2 and see if it a little slow going form rich to lean. Or you can just throw one in and see how it goes if it has been a few years it wouldn't hurt anyway.
1 SDL, 1 D, 1 TD, 1 Mog