ok, false alarm
i broke the piggy bank and got another Cnurse. air temp just before the rad is 22C and CPUburn (after an hour or so) is 39C. so the 17deg diff is back. my guess would be that the room thermometer i was using got affected by the sudden changes in air temp. we had freezing temps last night! phew
hey, at least now i can know my water temp as well...