Hesitation when coming out of a shift.

The problem i'm having is with my 96 G/A GT 5speed trans. when shifting from 3rd to 4th and 4th to 5th the car will hesitate and buck until the RPM's come up. The problem is alomst non existant in the lower gears.I have replaced the Ignition coil packs [several times], the ignition housing, plugs & wires. I have also checked the injectors and run cleaner through with the gas. The car has 113,000+ miles and this began at about 90,000miles. the problem was worse when I had a hole in the exhaust at the "y" pipe, after replacing the exhaust from the cat back the problem improved slightly. I also get some improvement with different brands of gas. after going through all of the listed ignition parts I replaced the fuel filter... no difference. could it possibly be the O2 sensor? Any suggestions would be helpful!!!!

yes. very well is the o2 sensor. change that and change out the plugs and wires, should be less than $100 to do and it should solve your problems. If not it could be teh cat or a vaccum problem