Cat2000 won't start after it warms up and is shut off- why?

I have a 2000 Catera that usually starts for me in the morning-drive it to work, starts after work but if I make a quick stop somewhere, shut it off and try to restart it, it tries to crank, then nothing. I have to wait at least an hour or so and then it usually will restart. I changed the battery last week and she started right up every time- now it is doing the same thing as before I replaced the battery. Took the car up to local part store and the tested battery and alternator and both checked fine. Did get and O2 sensor read on the emissions code. I have read about bad CPS on these Cateras but the car has never died on me while driving it. Wouldn't think it was the starter since I do get some crank out of it when I try- other times however, nothing. Thanks for any insight you could share. I am baffled!