I just had this same issue with my 99 GTP. I pulled the TB and scrubbed it clean. Scottydoggs is right, don't waste the cash on a new MAF, the junkyard ones typically work just fine and are a lot cheaper. Save the money for more mods. Anyways, after I cleaned my TB and MAF sensor (with MAF cleaner) and re-installed them all, my issue was resolved. Its worth a shot. If it doesn't work, then at least you have a clean TB and MAF sensor!