I just recently had a similar problem on my '08 Sierra with 5.3. I finally took it to the dealer after having talked to Onstar who told me it was a throttle position control sensor. That sensor apparently is apart of the throttle body so they had to replace the throttle body. They told they had seen it quite often and fortunately it was covered under the powertrain warranty. My truck is also tuned but I just swapped the units out. I also got a reduced engine power message so may not be the same thing but figured Id give my two cents..good luck!
