Odd temp issue - 240D

[Biodiesel300TD]

On my 240D I've been having an odd temp issue. Randomly my temp will rise up to near 100C, so I'll crank the heat on, and after just a minute or so the temp will come down to normal operating temp. Today it didn't even take that long. It usually doesn't have an issue, but every couple of weeks or so it will do this. During the removal of my veg oil system I replace the t-stat as well as the heater hose coming off the back of the head. I immediately had issues of not reaching operating temp which was solved when I cleaned the plug on the sensor in the engine block. Previous to the VO system removal I never had any temp issues. Does this sound like a t-stat problem? Why would it only do it randomly?