mk4 2.5 di slow cranking when warm

[zephyr]

This one is a bit odd, normally starting problems are from cold.

I have a 1993 Mk4 2.5DI.

When its (relatively) cold, it cranks over and starts fine. Once it has run for 20 minutes or so, if you turn it off and then try to start it again, it is really slow turning over and sometimes won't turn over at all. If you let it cool down for a couple of hours, its fine again.

I bought a new heavy duty battery for it and it was fine for a couple of days, now its back to the slow cranking.

I have a booster and if I put that on when its having trouble, it starts right up.

Alternator light goes out once its started so it looks like the alternator is at least doing something.

I'm reluctant to spend another £90 on a starter motor if its likely to be something else. Anyone got any ideas?