As title really.

On the road/track the car doesn't get hot, even pulling 7k+ in 5th, and is fine sitting in traffic etc as long as you like, but once you stop and turn the engine off, it bubbles over quite a bit- Regardless of this though, temp gauge doesn't show it overheating (it does work, leaving the rad fan off shows that!)

Immediate thought was the header tank cap, so changed it for an uprated one and its the same problem.

Removed water cooling to the turbo and the modine to see if that had an effect- Nope.

The header tank and radiator is smaller than a stock Cossie one (Engine conversion), but while it deffo doesn't overheat normally, will this be why it boils over when the engine is off?

There's no swirlpot, but I don't know if they're a gimmick or not anyhow.

Any ideas? Doing by head in to be fair.