when bottom end bearings start to wear oil pressure is lost (particularly at tickover because this is when its lowest) but...

I had this problem with my rst turbo, i took the sump off to check the pick up pipe (sometimes the strainer gets blocked) but it turned out to be a weak oil pressure switch. Changed this (only a few quid) and now its ok.

Speaking to the partsman @ ford he said he sells loads. So theres a good chance it could be just a cheap sensor.

Good luck
Pete