PO455 2004 Blazer

2004 s-Blazer with 4.3, 126K mi. The PO455 code for evap emissions leak kicks up about once every 3 days or so, and then I clear it with my code reader. I have already replaced the gas cap, so what else do I need to look for? Vehicle runs fine other than it has always idled a little rougher than I would like.

Large vacuum leak on the system.

Code sets when the EVAP system test for holding vacuum fails.