1995 Ford Mustang 3.8 V6 automatic. Transmission problems.

I have a 1995 Ford Mustang 3.8L V6 with automatic trans. It's been acting up lately, like when I'm driving at higher speeds above 30mph it seems to disengage itself out of gear, then I let my foot off the gas pedal for a little while and it re-engages back into gear. When the transmission is doing this, the RPM's fluctuate from about 2000rpm to about 8000rpm. It's almost as if it were a bicycle, the chain was falling off and back into its gear again. The check engine light is ON. Gas fumes come in through the vents when the heater is on, only when the car is stopped. The car also failed inspection in NJ for the idle, the reading for HC ppm was 1575 and I don't know why.I just replaced the throttle position sensor and mass air flow sensor. No difference. I'd like some help, any tips on what it could be, like a speed sensor, timing chain, o2 sensor? By the way the car has about 193,000 miles on it, but the motor was just taken apart about a year ago and got all new gaskets, head gaskets, plugs, etc. Thank You in advance!