1995 Buick Park Avenue Delay in acceleration

I've had my 1995 Buick Park Avenue for almost two years now. When I first got it in the summer of '08 I had the gas tank replaced and new spark plugs put in it the following winter. This past January I had to replace the battery. It's run very well for me thus far but now I am having some trouble with it and I'm about to take it (or had planned to) on a long road trip from Ohio to Texas.

Unfortunately, just this month I had to have a pulley and serpentine belt replaced and then last week my check engine light came on and my accelerator started lagging when I would press on the gas pedal. I took it in to have the problem diagnosed and I was told I would have to have my knock sensor and water pump replaced. So I bought the parts and had that fixed, and it was finished yesterday.

However, I think it may have been a misdiagnosis because while it drove fine enough last night and this morning, by the afternoon my check engine light came back on and the delay came back. Now, the mechanic reassured me when I first spoke to him that it -was not- my catalytic converter. But I'm not car savvy so I really have no idea what the cause of this could be. I know that there's a delayed response after I am stopped when I have to press harder on the gas to get it to go. It also offers a bit of resistance when driving uphill.

Does anyone know what else could be wrong? I'd appreciate any input, thanks!