Fieta Fun Downhill

Hi

Can someone advise me regarding the way ahead with a problem my ’98 1300 Fiesta developed over the last month ( this is a model on sale in South Africa with Endura engine) : When on a steep downhill in top gear the engine just quietly cuts out : (oil- and battery warning signs go on) Nothing else & it restarts but this is a worrying thing on mountain roads . It seems that it can be prevented by switching to a lower gear or continuing to just touch the accelerator.
I’ve only taken it in once and the mechanic says he cleaned the plugs and checked timing and idling. He mentioned something about a fuel pump relay being next ont the list of things to be attended to but I couldn’t follow much due to my lack of knowledge of cars and the noise in the shop. The problem remained unchanged.
What should I have checked out next , in order of costs to be incurred?

Thanks

Hey
My Fiesta Does Exsactly the same thing, High gear going fast, Stalls, restarts fine, I found out that the problem lied with in the pipe that led from my engine to my air filter, it was clogged with stuff that resembled baby poo. easy to clean and i suggest changing your oil sharpish.
Ford Fixed