Hard to believe the factory system wouldn't cool OK, even in a Texas summer. A fairly simple test would be to install a manual shut-off valve in-line with a heater hose (see posts here). If it works, it could be a permanent fix - just turn it off every summer. My guess is that you are getting hot water flow thru the heater core. Might also try replacing your factory automatic valve. If that doesn't work, you could consider switching to a HC refrigerant, which works much better and works with PAG oil (recall). But read a lot first since there are scary stories of cars maybe blowing up like in Hollywood movies, though no actual story I ever read.
