It will be the Universal joint. This is known on Fiesta .. in fact my old diesel used to do it .. when it got wet it was ok again.
This is tricky to replace as directly above the rack .. early type is now probably obsolete.
Later type new will be £50 ish at dealer now. Aftermarket available .. See here .. That's why the replacements are available!
http://shop.ebay.co.u...at=See-All-Categories
Stall would be obvious as a restart needed.
Pump does not give any symptoms like that.
Belts don't slip as very low load .. would be accompanied by bad pump noise.
Good guess on it being a Fiesta tho ; )