Car is slow to start off.

1994 ford escort wagon 90031 miles. 1.9 Liter engine When it is in the 80 or higher. The car starts fine and runs great after about 40, but before that it is slow, jumps, sputters. It seems like that engine is bonged down. We have changed the fuel filter. Had the fuel pressure check and it was ok. The spark plugs was recently changed. I was told that the catalytic converter could be clogged or that it could be my oxygen sensor. I don't know help please. Trying to get it ready for college.