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Hi this is usually a fuel supply problem but as you say it only happens when its hot its very unusual.
The photocell either produces a lock out Or the motor will just keep purging without trying to fire.
The transformer which is combined with the control box also produces a lock out.
yes its quite normal for the pump pressure to be low until the coil activates.
The Coil will normally produce a lock out but i suppose if its activating and the valve is sticking it will produce this fault, because no fuel will be getting through to the nozzle.
If your absolutely sure fuel has a positive gravity pressure to the pump at all times then i would try the solenoid valve first.
coil voltage should be 29 volts on start up and 3 volts when running but I didn't think it was a problem when its running. I am thinking you may have a sticky solenoid valve
The photocell didn't produce a lock out or shut the flame off? Then yes I would certainly try another one in there