HELP my campervan keeps stopping

I am having problems with my VW transporter 86' and was wondering if anyone has had similar.
She appears fine when bimbling around town but the minute she has to sustain 40mph + she completely cuts out and dies. She seems happy to restart but alas 10 mins down the road and you hit 40 she does the same thing. This has happened several times now so I took her to the doctors where they cleaned fuel lines and filter, cleaned the carb and reattached some loose wiring. Although she sounds wonderful she is not happy. Has anyone had similar or can anyone shed some light on my very annoying problem.

Thanks