05 Equinox. Did a head gasket job and everything went smoothly. Started it up when finished and it ran fine. After reaching operating temperature, the RPM's will start to surge until it stalls. It starts right back up and will happen again a few minutes later. No codes present and it's smoking out of exhaust still, only not as bad and it doesn't smell like coolant. Smells more like it's running rich. Also seems to be a slight backfire when I rev the engine. Any help is greatly appreciated, thanks.