Dear all - I'm only posting this as I had this happen to me.
If you don't already, I would look to calibrate your kit more often!!
I first suspected something wasn't right when I was racing the GT86 at Barbagallo... besides being a bloody menace
I was hitting Kon's corner nicely and looking to pass people down Supercharge Straight... I so had a good exit speed, but up the hill I just lost pace against my fellow racers and they just started to accelerate away from me... what the hell??
So this morning I had a go at the lut generator and wanted to give Spa in the Audi a go... this was when I saw that my throttle was going the strange fluttering effect at the high revs down the straight...
I recalled it was doing it when I was trying that combo out right after AC1.5 was released, but didn't think anything of it as I smashed my PB by ~2secs.
"Phfft... nothing's wrong... I just smashed my PB!"
Anyway this morning I checked what my accelerator was doing in game under settings and it was fluttering between 96-97% full press!! What the hell??
Ran Pedal calibration out of game, ran calibration in AC and tried Spa again in the Audi... throttle looked and felt awesome down the main straight... even was getting up to the pinging top 6th gear just before I had to break at the end of Kemmel straight... this feels good...
Smashed my PB again by over a second... stoked... but know there is more there...
tl;dr - calibrate or at least confirm you are getting full break/accelerator range often.