Weird Problems at 2000 RPM
 

For the last month or so I have noticed that at 2000 +/- 200 RPM, my truck hesitates, the lights flicker and the battery gauge almost pegs out. When I am decelerating, it isn't an issue, just acceleration. Notice only when at that RPM range, other times its fine. Had alternator checked, said ok. I have had an ECM put on back in July, but just noticed the problem recently. Someone said maybe the ECM is putting out the wrong voltage.. Any ideas? Thanks in advance.

Clean your battery terminals and check all your grounds.