I'm not sure there's a thermal paste that
doesn't freeze at ln2 temps, ln2's kinda cold
I've been having excellent luck plopping a bunch on, squishing the pot on top, clamping it down, and then running the cpu at normal to slightly elevated volts and light load till it's at 60-70*c, then starting with the cold stuff.
The patterns when i remove the pot look great.
That is, of course, dice.
EDIT:
Thermal expansion and contraction of the cpu would be my guess, and the movement breaks contact with the pins ever so briefly.