"Upgraded Throttle Body"? What did you put on the car? If you put the beefed up TB on without a tune, I'd expect you to have trouble.

It is a bad idea to mod an engine until you have it running properly, that way, when you do the mod, you know if the mod caused the issue. As it is now, you don't know if the mod did this or if you are still working on the original problem. Put the STANDARD part on and see what you have. DO NOT mod the engine until you have it running properly with the standard parts and configuration. Once you get that accomplished, THEN it's time to start trying mods.

Stalling on opening the throttle like that sounds to me like a disruption or variance of the Air-fuel ratio, which, if you put a different TB on than the ECU expects, would be possible.

(Yes, a screwed up MAF can also contribute to stuff like this. Maybe order the OEM MAF as well. Also, not sure why the Crank sensor was recommended, but it's a good idea to keep a spare with you , they fail without warning. FCP, German Auto, Needswings, AutohausAZ are all good sources. OTHER than Needswings, tell them you need a CPS for a 2003 SLK320. DO NOT search crossfire parts anywhere but Needswings - only Rob at NW knows what he's doing insofar as a Crossfire.