80ff cutting out ?

central heating fires up but keeps cutting out then starting up again.
when i have taken the front panel off to expose the flame the unit runs fine,when the panel is put back on as soon as it is in position it starts cutting in and out again,when i leave a 20 mm gap at the bottom ie fix the top in place.is this a pump issue,have blown through the pipes from the pump to the fan.cheers in advance for any advise mike