need help, car not running right
Now, last week it got pretty nice out so i decided to take it out for a spin. It started a little hard because the battery was a little dead, which i expected and let it run for a little while to warm it up. I drove it around for a little while without beating on it because it had been stored for over 4 months. Then when I gave it a little gas it would run ok untill about 3500ish rpms and it would misfire again, not as bad as when it had the tune, but still really bad complete loss of power. Also everytime the engine would go back down to idle after being under a load it would go down below 500rpms and shake like it was ganna stall and then catch itself, it would do that a couple times then idle fine untill I put it under a load again. Revving the engine in in neutral seemed fine as well.
After that I chaged my egr valve cause it was sticking a little and that did nothing. Resetting the pcm and loading the stock tune on again didnt do anything either. Would this be somthing wrong with the computer? It never did anything like this before I loaded the tune on. Im awaiting an e-mail from where i got the tune from, but I'd like to get some imput from you guys too. Thanks in advance and sorry about the lenth.
Its an M6, it does happened everytime its under a load but nothing before 3500ish rpms, runs fine before that. It does try and stall when going down to idle after a load whether I bring it that high or not. There arent any codesthat i know of. If it is the opti, is there any way I can make sure without having to buy one first? Thanks for the help.

