Dell Diagnostic Indicators  In case needed for reference, 760 goes back to 2009. Below is what Dell Diagnostics had to say and I wanted to present the options.  I also still think your Q8400 might be faulty as fault only occured with it installed.
Hello I upgraded my dell optiplex 760 with a q8400 and it does not boot, seems to be some sort of boot loop. Lights 1 3 and 4 are lit and the pc powers up and shuts down. I have tried reseating the q8400 with no luck and replaced it with the old Pentium and it booted. Thanks for any help
Dell optiplex 760 q8400 not booting