Car stumbles HELP!!

[coolguycooz]

Ok so ever since i got my cold air intake and my catback and muffler ive been using 91 pump gas just cause i thought it was better. Now i put back regular 87 and for some reason i dont even know if it started now im not quite sure but when i like lets say im going 40 and i shift in to 5th gear my RPMs would be like 1900 and i would kind of step on the gas it would feel like my car is kind of bumping/stumbling and i remember back in the day i would do that to kind of coast but not slow down and save gas and i was wondering is this happening because im using 87 gas a gas shortage or what? i dont even know its just kind of fruity cause it only does it before 2000 after thats it normal.

[helliviknow]

sounds kinda like your boggin it out to me.. i dunno i think a cat back might mess with the low end torque and raise the low end hp higher upt he band but i dont think it would make that big a diference.. this is a nt right? if so i have alreays run 87 in mine np and if its a turbo well.. i think ur posed ta use the premium gas..

[ashah000]

When was the last time you changed you spark plugs? What kind were they?
If you are turbo then you need premium gas.

[dankid]

I have a similar problem with my car. I own a 96 GS N/T. Everytime I put the car into 1st gear and start driving my rpm's drop for a split second really low but then go up. If the car's cold it happens really bad to the point where it almost shuts off. I recently put in some new Platinum Quad spark plugs and a new set of 9.3 plug wires. I don't know what's causing the problem.