Problems starting 90 Buick after car is warm

HELP please!! i am new to this forum (actually found it while doing a search on this problem). This is on a 90 Buick Regal, previously ran fine.

The first time it happened I had stopped at a store after a 45 minute commute from work, ran in, ran back out and car wouldn't start. It cranked over fine but engine wouldn't catch. I came back the next morning and car started right back up...at the time I blamed it on a 30 degree warm up during the day and maybe ice crystals melting in the fuel line or something...the next morning when I started it it was cold again.

Now this just happened again yesterday - coincedentally at the same store. Started fine leaving work, I stopped for no more than 15 seconds (store was closed), and then it wouldn't start again. Once again, the next morning when I went to get it (from the impound lot as some nice tow guy had come by and made himself $150), it once again started right up.

Drove it to work today, and just out of curiousity when I reached the parking garage, I turned it off, waited ten seconds, and tried to start it again - sure enough I couldn't, but I figure by the time I leave work today it will be fine...

Any ideas? I'm at a loss...it's not going to be work I do but would like an idea of what's causing this before a garage tells me I need thousands of dollars worth of work because I know nothing about cars! :)