Do you know if your CEL (check engine light) is on? If it is, get the code read by a fellow CEGer with a scan tool or go to the dealer and pay to have it done - sup'm like $50. come back here with any codes that are read.
It is possible the problem is a sensor that has become heat sensitive, or the catalytic converter at the front of your 4 cylinder engine has become clogged (not likely), or you're having mis-firing problems when the car warms up. You may indeed, as you suggest, have a problem with the fuel pump.
When was the last time you changed your fuel filter, air filter, plugs & wires?
