Purge Valve Related to Catalyst Monitor?

Ok I have a code P0430 and it confuses the hell out of me. 1996 Suburban, 5.7 liter vortec, auto, 4X4. I had this code a while back, but turned out I bought a faulty o2 sensor at checker auto. I said to heck with this stuff and bought 2 new AC-Delco 02 sensors for bank 2. Everything was perfect for about a month, other than the occasional p0441. When I put my code scanner on, everything checked out, all system checks were completed. Yesterday code p0430 popped up again, after i cleared it, the code scanner gave me a yellow light and the "Catalyst Monitor" light was flashing. This was new. I no longer get code p0441, but only p0430 shows up after about 30 minutes of driving. I switched around oxygen sensors just to test it, and the same thing. I also unplugged the evap purge valve just for kicks, and it does the same thing, no code p0441 but code p0430. I realize that p0430 is catalyst efficiency low on bank 2, but would that cause the catalyst monitor system to never complete its cycle? I am going to borrow an o2 simulator from my friend so I can confirm or deny a bad cat as the culprit. Anyway the question here is, can a bad evap purge valve cause the catalyst monitor not to finish its start up check, and throw a code p0430? Thanks for any help I really need some right now.