Hot water furnace problem.

I have a New Yorker hot water furnace that provides my baseboard heat and my domestic hot water. The problem that I am having is the burner comes on when it should and shuts down when it should and runs within the limits set on the aquastat control. The problem is after the burner shuts down upon reaching the proper temperature there is a delay and then it starts again and runs 2-3 seconds and then shuts down. I have a burner service contract and the system is maintained under this contract. The burner service people cannot figure out what is going on with this problem. They have replaced the aquastat control and checked all the settings to insure they are properly set. This only happens during cold weather when it is being used for heat and hot water. In the summer when it is used for the hot water only, it runs ok. Has anyone else had a similar problem or any ideas of what could cause this problem? Thanks in advance for any help or suggestions that might solve this problem.