If it was fine before...and now is stumbling, with same tune and boost, then I suspect something like MAF meter just dirty and needs cleaning or possibly the above suggested plugs, wires, etc...

I would check the plug gap and change the fuel filter

under boost

fuel pump...

Is it just stumbling or actually feel like it's cutting completely off and right back on again. If it's the latter, then it could be just a bad connection of the chip. :shrug:

Dude u ask for help but you provide no detailed info about the car. U r asked questions regarding the issue and u provide short responses that give us little insight to your problem.