So you're saying that during a Prime95 test run, with the CPU usage at 100% on all 8 cores (which I assume you are monitoring with something, no mention of that...), you see the CPU multiplier change between 17.5 and 7? That is with 8 Prime95 "workers" running simultaneously? What are your core temperatures like during the test when at 100% CPU usage?
Yes, the multiplier should stay at the highest value during the test, but can drop if the CPU is over heating, so throttles itself down. You're saying during the Prime95 test that all your cores are in the low to mid 40's? According to what monitoring program?