Hi Duckstaped, welcome to TSF. Sorry for the delay, I don't know where the Games team are hiding.
What make/model is your PSU? Check the label on the side of the PSU - how many amps are listed for +12V? If it's less than 26A, then you're underpowered for a PCIE card.
Enter your full system specs into this
power calculator and set Capacitor Aging to 25%. Click the Calculate button, then add 30% to the total to allow for PSU inefficiency. Post back with this figure.
Check your temperatures and voltages with
PC Wizard at startup with no other programs running. Take with a screenshot from the 'Voltage, Temperature & Fans' window. Then open Task Manager and click the Performance tab. Leave it open on the desktop and take another screenshot of PC Wizard and Task Manager
during gameplay when it starts to lag by pressing ctrl-alt-del to switch back to the desktop. Post back with both screenshots so we can see how your system copes under stress.
Are your chipset drivers, device drivers and DirectX all up to date. Any yellow flags in Device Manager?
Leaving the case open to reduce temperatures should only be a temporary measure while testing. It needs to be kept closed so the front case fan can pull in cool air from outside, pass it over the motherboard and components, then exhaust the hot air via the rear case fan. This recommended airflow path is destroyed by opening the case. Do you have 2 120mm case fans, one at the bottom front of the case and one at the top rear of the case?