Golf Clipper Con misfire under load (fuel starvation)

Hello Chaps, this week i have had an issue with my cab she seems fine on light throttle but under hard acceleration or motorway journeys she has a loss in fuel and get a miss fire and grinds to a halt not good on the M4. she has a new filler neck and tank as well as having the lines cleaned out and an inline fuel filter fitted, she also runs a manual choke weber carb, since the tank etc went in she has done 2500 miles and has been ok, is it common for the mechanical fuel pump to become weak, this wasnt replaced but looked new.
Can any one help.

Cheers Chalk
1800 golf clipper con.