I'm a bit late jumping into this one (I've had a computer virus for 6weeks now !! another story!)
If the solenoid coil doesn't do the trick it could be possible that air is getting into the oil supply pipe, is there a tigerloop (de-aerator) on the system, is the oil storage tank above or below the boiler? I am wondering if having the heating turned up high is keeping the burner on for longer periods, I suspect that the problem occurs while the burner is idle, possibly due to leakage of air into the oil linesa under suction while at rest?
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