Might be due to a hole in the exhaust, I've changed the middle two sections of mine when I bought it recently, but still have a slightly louder exhaust note (more bass/deeper), popping and slight drop/dip of the revs if you rev the engine before it settles at around 850rpm. I will replace the rear section of the exhaust as I think this is the cause of most of the issues, and the slight flatness as you described.
Then after that I will check if I have; or will change to the 'blue' temperature sender as per the tuning guide on this site for the DIGIFANT engine. Then have the car checked over when the cambelt, fan belt etc and the clutch cable are replaced in the next week or so. Quoted around £280-300 for all the work, plus the exhaust.