s10 starter problems

[Porkchop]

I have a 99 s10 and when the engine is warm like after at least a ten minute drive the starter won't turn. I get nothing when I turn the key to the start position, everything else works. When it is cold everything works fine. The battery is in great shape, the alternator puts out okay, the starter relay is fine, and the starter is fine. When I run a jumper wire from the positive of the battery to the small terminal on the solenoid, it turns over just fine, but the key still doesn't work. Anyone have any ideas on what to check?
thanks for any help