my wife has a 91 f250. Lately she has noticed that first and second gear run fine during acceleration, but when it hits third gear it lags like it is starved for fuel. It also shakes a little too. I talked to my mechanic today and he said it might have something to do with the exhaust, that the muffler is clogged up. I know enough to be dangerous to my engine, but not enough to fix a major problem. any ideas?

Start by changing your fuel filter.

that was the first thing I did. I did that about 4 weeks ago. The last time I changed the air filter was in november of 04. She says it has been doing this since feb 05.