Pontiac Repair: 1995 pontiac firebird, pontiac firebird, auto parts stores

Question
I have a 95 pontiac firebird v6 3.4l once i get about 45 it vibrates really bad but once i let off the gas it rides as smooth as it should. So nother words when i press on the gas and get above 45 it vibrates but when i let off the gas theres no vibration can u please help. I have already replace the egr valve and now im going to replace the spark plugs and wires and one more thing when i get on the freeway and the first time i step on the brakes the check engine light comes on but once i turn the car off and back on the light does not come back on. So theres no way to run the codes on it cause i have and it comes up with no history this is done once the car is turned off so the light does not come back on

Answer
Hi Keri:

Well I think you may be on the right track with plugs and wires. that enegine was noted for wire issues. also look at the tops of the coils where the wires attach if any of them are rusted looking or black replace that coil pack. As for the check engine light if the lamp is on for any extended period of time it should set/store a dtc(S) stop by any of the big box auto parts stores and have them scan for codes most of them do it for free. if it doesn't store any next time the light is on stop by one and don't turn the car off and hopefully thye can see what codes are stored without knowing the code # there is not any direction to go in.