Car stumbles HELP!!

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.

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..

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

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.