immersion thermostat cutting out. Air?

[bonym]

Hi,
Was wondering if anyone has any ideas about this.
A customer called me saying his immersion heater keeps going to cut out and he has to keep resetting the overheat switch, he also complains that water is not hot enough, Its a vented cylinder by the way with a 27inch immersion heater, its a newish immersion and the third thermostats. He said on the phone he thought there might be air in the top of the cylinder making the immersion overheat. I explained that it wasn't really possible as any air would leave via the open vent pipe. When i arrived at the property i discovered that the vent/supply pipe offset from the top of the cylinder was instead of being horizontal or rising slightly was dipping to the vertical vent pipe. Sorry for the long winded intro but my question is does anyone think that air could be causing this problem, and if so where could it be coming from.
Cheers