CAB NY

if anybody have a idea i first notice this problem after i installed the new mufflers but not the same day and i don't drive the car everyday so i don't know if its a everyday problem

1. i notice it more on the highway when i driving steady the RPMs will stay about 2/3k but when i release the pedal it wont drop down but if i tap the gas 1 or 2 time it will drop back down but this i not all the time that this happens

2. if start from zero and gun it this happened about 3 times the check engine light flash a few times then went out but 1 time it stayed on and had it checked and reset but i think it read all cylinder missed fired

if i explained it right any ideas

Thanks