Baxi Bahama 100 Start,Stop?

My boiler starts and stops. I have had most components replaced. The pressure switches are fine and both heat exchangers are clean. I have checked the ignitor sensor wires etc,fine.
While it was starting and stopping i took the cover off the airbox and this solved the start stop, WHY??
I know that the airbox should be closed so i have done so.
Q. Why does it work fine with cover off????