Its not an earth problem, this is very common on rs turbos, cvh,zvh etc.

Its called heat soak, the manifold and down pipe get so hot the starter motor cant handle it, there are a few ways to remedy this.

Heat wrap your down pipe, get heat shield for starter but they are hard to find

Upgrade your starter to a zetec or 1800 diesel one, if you do upgrade it, you would still benefit heat wrapping ya down pipe.

hope this helps