Hot Water Heating system problem

We have a Burnham series 2 hot water heating system. It was installed about 2 years ago. The past week we have been having the issue where the hot water does not circulate the baseboards after a period of time. I first noticed this when the thermostat was significantly lower than the desired set temperature and I did not hear the water noise through the baseboard. I called a plumber and he hit the reset red switch on the Honeywell control device and the blocked vent switch. After a few seconds the water starts circulating up to the baseboards. This has been going on at least 3-4 times this week. The plumber claims most likely its a poor blocked vent switch. We end up replacing that and we are still experiencing this. His next guess is that there is a blockage in our vent/chimney? The chimney was cleaned prior to the new heating system installation 1.5 yrs ago. Anyone experience anything like this? What would cause the hot water to stop circulating and resetting or hardwiring the block vent switch would fix it temporary? There is nothing wrong with the physical burner itself since the pilot light is on and its heating the water to the correct temp.