I have been fighting the same issue. I narrowed it down to the thermostat housing but so far I haven't been able to solve it. I have changed the housing, installed several gaskets using different sealants and tried different torque settings. Every time it seals for a while but eventually starts leaking again. Sometimes it can appear to be fine after a ride but when I check back a few days later "it's baaaaaack".
I have checked the housing to make sure it is flat, That seemed to be the problem with the original one.