2000 Tahoe error code P0440

My "Check Engine Soon" light came on last month. I went to Autozone and had the code checked - it was P0440 - what I've heard referred to as the "loose gas cap" code. Since I had recently replaced the fuel pump, all kinds of bad thoughts raced thru my head, fuel lines coming undone, car going up in a ball of flame, etc. I looked over all the connections and all seems well, no noticeable leaks. After about 3 days, the Warning light went away, and I assumed divine intervention had corrected the problem. the Warning light came back this morning for about 10 minutes, and is now gone. Any thoughts of what I can do myself. I hate to go to the dealer and I'm a reasonably handy guy.

check for dirt/mud in the evap purge solenoid and charcoal canister. most shops have a smoke machine which will find any leaks pretty quick.