Electrics: Hot start issue, starter won´t crank when hot

[FlashGordon68]

Since I bought my 2006 D5 XC90 1 year ago, I'm having issues with a hot start problem.
Previously the symptoms were a long cranking time when engine was hot. No issues when cold, so I thought it couldn´t be the battery.
I've already replaced the starter and the battery in separated steps, and in each step it seemed it was solved, but after a while, the problem is back again.

Now, it's even worst. After driving a while if I stop the engine, I have to wait a litlle (10-15m) to cool it down before I can attempt to start it again. The starter won´t even turn, and it seems it makes a short-circuit to the battery because it drops to 9V.
I don´t know if it's possiblle, but I think I can have a heat soak problem with the starter. I haven´t tried yet using a booster cable directly from the battery to the starter suspecting it could be a corroded cable but I thing that would also cause problems when cold. Is it possible that a heat shield around the starter could prevent this from happening?

Thanks