Also check the slave bracket, some of these tend to bend over time. Here's a link to a short thread:
Clutch Slave Cylinder Bracket
But, if you search "slave Bracket" you will find more. Mine is bent a little, but I just live with it. Also, my TKO 500 is harder to get in reverse, but I thought that was typical with all trans. Since yours varies with temp, I believe a good flush will solve your issue. Slave cylinders are cheap tho, the cross reference for mine is Beck-Arnley 072-8342 (fits 87-93 Mazda B2200)