'96 Cav. Automatic Transmission Issues

[MrMister]

I have a '96 2.2L Cavalier with an automatic transmission. The car has about 275,XXX miles on it. Recently, I have been having problems with the transmission. Basically, the transmission doesn't shift unless I release the gas pedal and press down on it again. It always happens around the 20-25 m.p.h. mark. If I don't release the gas, my speed doesn't increase much, i.e. it goes up to about 30 m.p.h. and I hear the engine revving hard. Once I do the gas pedal thing though, it accelerates well. Any ideas on what could be causing this problem?

[Blue Bowtie]

What is the condition of the vacuum modulator on the front of the trans? What about the vacuum line from behind the TB to the modulator? Have the trans oil and filter been changed recently?