heads installed-car hesitates sometimes

[sick95gt]

I just had my edelbrock performer heads, custom cam, rr, pushrods etc installed. The car's power is great for the majority of the time, but every once in a while, I will downshift and the the car does not go anywhere (ZERO acceleration). It's as if the car has a rev limiter during that time. Then when I come to a stop and take off again, all the power is back. My O2 sensors are stock from 95 with 140K miles on them. Could this be the problem? The injectors are brand new, 24# calibrated with a 76mm maf. Timing was set at 12*. I replaced my maf sensor not too long ago and also cleaned it not too long ago, but could that maybe be the problem? I am going to get a tweecer or dyno tune here very soon. With a stock computer, would this cause intermittent power like this? Oh, and the check engine light flashed on for about 30 seconds and turned off and has been gone ever since. Will the code still be in the memory? My car was in the garage with a blown headgasket for about 6 months. Could it be bad gas? My tank was almost empty and today I just put a new tank with 91 octane. Any ideas?