Snapper Z1804k zero turn transmission slips only when hot.

[Memphisvette]

I have a Snapper Z1804k zero turn transmission that slips only when hot. When it cools down it turns and goes like new, but , after 15 minutes it begins to lose response until it finally won't move. Cool it down, and it's good again.
I have changed oil and filter and belts. No change. Anybody else have this problem? What causes it?

Scott