Full engine rebuild 15hrs agoDon't want to sound like a 'doomsayer' but it could be a sticking piston, as mentioned in other reponses.
What are your compression readings?
We had the exact sort of problem and had to get a rebuild.
We did the whole engine but you may only have to do a top end.
Pull your raves and shining a torch into the chambers check and see if there is any piston damage-scrape marks etc.
Goodluck hopefully this is not it!