Re-installing the CPU in your MB probably wouldn't help, unless there was a loose connection or corrosion on some of the pins, or something, which reseating the CPU might fix (unlikely). Replacing the CPU might be safer than leaving it out on the table, though.
You would have to either install your CPU in a different working computer or install a known working CPU in your MB (preferably both) to isolate whether the problem is with the MB, CPU or both.

If you can not get your hands on another compatible computer (that you can take apart), any decent repair shop will have test systems they can swap components in and out of. You mentioned the MB was still under warrantee, but how about the power supply and (more importantly) CPU? If a shop replaces something under warrantee there shouldn't be a cost (though you should check first in case they add shipping charges or something) but there would probably be some kind of diagnostic fee for out of warrantee components (ie an OEM CPU may only have 90 days coverage).