I suggest you ask Nick to disassemble and clean the pump thoroughly before replacing it. It's a fiddly job, but no problem for Nick! I had a similar experience on my car, when it would very occasionally hit a wall and not accelerate further. It took Nick a long drive one evening to experience the symptoms for himself, but it turned out to be dirt I the pump, probably from a bad lot of petrol. I saw the residue in the pump for myself and it cleared the problem completely.
Julian.