When pedalling in higher gears, the crank or cassette seem to completely lose grip for a second, then grip then lose it again. I could probably pedal all day in 1st and any front ring but when start to go in 6/7/8th it will do it.
This is not a gear change problem, all gears change perfectly and when the bike is on the stand everything works perfectly.
In order to try to cure the problem, the front and rear derailleurs have been renewed including cables, the chain renewed, the derailleur hanger also, but the problem remains. I tried swapping the back wheel with known good one off another bike to eliminate the cassette, but the problem stayed the same.
The front crank has a tiny bit of play but the teeth look ok - none obviously more worn or broken than others - the biggest ring on the crank has been bent and straightened, not perfect but it has been working fine previously like that, plus the problem exists on the two smaller rings too.
Any help appreciated