Leyland Daf 1995 Horse Box Cutting out

has it ever had a cutoff switch wired into the pump harness?
if it has had one as an anti theft device by some previous handy horace owner,then they will have probably used a scotchlock which will have frayed the wires enough for them to start arcing and shorting ever so slightly causing the pump to choke itself.
i had similar to this on an old turbo diesel transit and after getting quoted £3500 by the dealers for a new pump that the computer diagnosed,it ended up getting a shoogle test one wire at a time and the fault showed up.