OptiPlex GX 280 Shuts down during startup

(System belongs to an older lady) GX 280 freezes moments after the Dell Splash screen comes on. Error message "Previous shutdown due to thermal event". Press F1 to continue Selecting any option and system immediately shuts off. I have removed and reseated all drives, memory, and cards. I have removed the cmos and restarted. Case is clean and very little dust. All fans are working although the main fan has run at full speed but that has not happened today. I do know the onboard video went out and this is why she called me for help. After I installed an new video card for her the video worked again but this is when I came across these other issues. On startup the power light comes on, there are 2 beeps. Hard drive light comes on then off until the system 'continues' or you press F1, then blinks once and system shuts down. The diagnostic lights on back. After flashing various combinations they end up G-G-G-Y. Then when you press F1 they are G-G-G-G. then when you start normally The system shuts down and all lights go off. Hopefully someone has an answer. I am trying to keep this lady from having to get a new system.