I have a 1992 Chev. k1500 silverado. Frequently when I go to start it up the starter just spins a time or two before finally catching. I had the starter tested & it checked out ok @ the parts store. What should I check next?

Sounds like the bendix drive is not kicking in.  You can change just the bendix dtive or it is easier to just replace the starter. The temperature has a lot to do with it not engaging and that is probably why it worked ok at the parts store.