Can't figure out what is going on and neither can the dealer technicians... First background on the car, it is a W211 (2004 E500) with M113 5.0 liter engine and "K3" Kleemann tuning which consists of supercharger, cam, full exhaust and of course ECU tuning.

Basically the engine will occasionally start running rough, like really rough idle, erratic revving, and major loss of power. The first time it happened it actually threw an engine code, took it in to dealer and they replaced the MAF sensor, after that ran perfect. But then a few days later it started doing it again, but with no "errors" at all. Took it back to dealer, and once it got into the shop/turned off and started up again, it was running perfectly and no codes, no problem data at all on their scanners. The dealer tech thinks that the Kleemann tuning is preventing the ECU from registering what's wrong, like aftermarket tuning often tells it to "ignore" the source problem, hence no codes. I am not sure on that, it's just their guess. So it will go for long periods of time with no issues, then one day it acts up. So I just park it, wait a while, then later it starts up just fine and runs perfectly.

The most recent incident was a few days ago, and when it happened this time the dash displayed "ESP Malfunction - Visit Workshop!" (that was the first time it has given that error). So I limped home, parked it, started the next day with no problems whatsoever and the error code gone.

So I'm at a loss, it SEEMS like some kind of ECU or engine management or sensor issue. Does not seem like "mechanical" problem since most of the time it runs fine, it just acts up intermittently - which leads me to suspect something electronic or sensor-related.

Any ideas what it could be? Much thanks in advance for any input!