CPU temp hits 80C then cools back to 55/60 [memtest]

I was running memtest on an LGA2011 system that seemed to have memory issues.
I noticed that when memtest starts, the core temperature is up to 80 Celsius.
Then it drifts back to 60 (with full 8 DIMMs). Testing only 2 DIMMs at a time
it goes lower to 55 Celsius. This is single-threaded test. So what is
memtest reporting? Just the active core, rather than average of the die?
This PC has a water cooling system. There is no model number on it, so I
can't determine what the thermal capacity of this cooler is. The radiator
fans seem to be going at fixed speed.
CPU is i7-3820. The RAM speed is 1600 MHz with CAS 10. No overclocking is
set in BIOS.