[Bobcat]

In my mind , the probable reason you aren`t getting up to temp. is that our radiators etc are all sized to handle max hp in a hot environment ... i.e , temps in the upper 90`s ... at least in SC , and you are not putting out any where near max hp and therefore heat when cruising ... and your ambient is a lot lower now . I`m also having the same problem now that it has cooled off here ( upper 60`s to low 70`s ) . I also have an aluminum engine , so its heat rejection/cooling ability is pretty high . Sounds like you have more heat rejection ability now than you have heat/BTU`s .
My solution is the same as yours ... block off part of the radiator . I also thought I had a thermostat problem , but really think that it is what I said above .

Bob