Hi guys

Went out in the turbo last nite, racing my buddies fiesta st, gave it a blasting up a mountain road up by me, car was probably flat out on boost for no more than a minute, and when i got to the top the engine was knocking and slapping inside like id popped a piston. Really bad. Ive blown a piston before in a bmw and this sounded exactly the same.
Gutted as i was, just shrugged my shoulders and began rolling it back down to the house. However, seeing as the brakes dont work unless its started, i let it tick over and freewheel down to the bottom (about a mile).

I got to the bottom and knew i had to drive it a little bit over another brow to roll home, but when i did the car suddenly started normal again! drove it easy for a bit and it was as if nothing happened?

Why did this happen?

Also, i decided to take it for the nightly spin anyway, and although it had a BIT of abuse throughout the nite, didnt play up again until right at the end, when after around 20-30 seconds on boost it started knocking and banging again, serious engine failure sounds. As i did the first time, i knocked it off for 5 and let it cool, and when i freewheeled it and put it in a high gear it started acting normal again.

Its sounding normal again now, but i dont want this happening on a regular basis. Why is this happening, what do i do to stop it and do i need to have it looked at now rather than later?

Cheers in advance cos i know theres plenty of ppl who know a thing or 2 here.

p.s. the car is the usual 180 (chip, exhaust, knN) plus a saph turbo running on around 10 psi boost. 5w50 oil.