Sorry to here you've got a gremlin somewhere
The only time I experienced anything like that was when I had my Revo stage 1 map put on, I had the same symptoms as you and was very annoying, that was when Revo had just released it and was about 3 years ago now, It took them a few months to bring an update out for it, but when they did it totally cleared the flat spot,
When my DV's have failed in the past it didn't feel like that it just felt underpowered and a bit like a Normally aspirated car,