91 Thunderbird Idling and Performance issues

Car:
1991 Thunderbird 3.8L V6 (NOT SC) 39,945 miles


My T-Bird recently blew a head gasket, and before I got it fixed I drove it about 100 miles. Since I got it fixed, it hasn't quite worked as well as it used to. My mechanic replaced many of the parts inside the engine with OEM or better parts when he found out they were broken.

My problems with the car are
1. It doesn't climb hills like it used to, and I have to push the pedal to the floor (I live in a hilly area, and I'm not content to slowly accelerating uphill).

2. When I push the gas pedal down slowly, the engine hesitates (almost stalls) before accelerating.

3. While I idle in gear, every 30-60 seconds the engine ALMOST stalls for a split second, then picks up normal again.

Problems 2 and 3 were present before the repairs, and my Mechanic reduced the severity of these problems by replacing the spark plugs and wires, but the problem never went away completely. The problem got worse after the head gasket blew.

My Mechanic claims that there is nothing wrong with the car, and nothing that he can do, but given the symptoms I know this to be untrue.

My thoughts were that perhaps driving the car like this messed up the cat converters, but a pressure check showed that they were working fine @ <1.5 PSI. Someone told me that driving the car with a blown head gasket messes up your O2 sensors, but before I spend the money on new ones I want to know if this is true, and if this might be my problem, I don't have a voltmeter, so I can't test my sensors.

Please let me know what my problem might be.