I had a similar problem, the steering would go stiff intermittently while turning when the suspension would compress during colder temps. I verified pressure coming out of the pump were within spec everything appeared to be sealed at the time, so I flushed out the system. The situation improved some, but it would still happen occasionally. Winter came to an end before I fully resolved the problem and the issue didn't present until the next winter. During this winter I had a bearing failure on the engine after being at a winter driving HPDE. While the engine was in the process of being replaced I noticed fluid seeping from the inner tie-rod boots. Removing them showed that the seals were leaking and I replaced the rack at that time. Since replacement of the rack the problem has never come back. The only thing I could come up with was that the leaking seals were either allowing air into the system under those conditions or would drop pressure. Not to say this is your problem, but it seemed to be the fix for the issue that I had.
