99.9% of the time car runs nd boosts great, really pulls, but ocassionally if Im putting my footdown a bit, Ill go up to 3rd gear, it will start to boost, and then just stops, doesnt drop boost, just wont go any higher. but I can de - accellerate, then go up through gears again and its fine

Hope this makes sense as its probably not the best explanation LOL

any help greatly appreciated