During that 8 hours, did your system crash, explode, send up smoke signals, make loud annoying noises you have never heard before, run out the front door screaming, or just melt into a puddle of foul smelling ooze?
No?
Then I would says your system is functioning quite well.
Prime95 does things to your computer that no other program will ever do to it If it can run for 8 hours without dying, things are most likely in good shape. Why Prime95 says 100% and task manager says 80%.. who knows? The real question is, "Is your computer functioning well?" If it is, use it, enjoy it.