hi guys new to ford ownership , bought a 2004 ford focus tdci , love the car but have a problem with it , driving along and every now and then car will drop in power , glow plug warning light then starts flashing , if i drive on it runs down on power and won't clear itself until i turn off the engine and turn it back on then it runs perfect again for another while ( could be 10 mile could be 100 mile) filters have been changed and its been serviced ford diegnosed blocked filter, when i told them filters have be changed they said its probably injectors and if not that its the injector pump they seemed to be guessing tbh. i can't see how if its fueling , flicking the ignition off an on lets it run ok again . Any ideas and thanks in advance

most likely is injectors or fuel rail pressure sensor both are common on diesels

just noticed it does it after its been sitting idiling or running in heavy traffic at very low speeds for a short period of time

Common fault with tdci engine is injector failing..
Bad design and expensive to replace

I'd say pressure sensor too.