Oil Light Flashing

Anyone have any ideas?
I have a landrover disco 200tdi, L reg. I had a recon engine fitted approx 25000 miles ago. Recently had a service and now have a problem with the oil light flashing. This only happens when the engine is put under pressure ie 10 miles on a motorway at a steady 70mph. As you slow down to a stop the oil light will start to flash until the clutch peddle is depressed, then it stops, lift up and starts again. Do a few miles at slower speed problem stops. I have taken the car back to the garage and they changed the oil pressure switch as that was faulty. Still hasnt solved the problem. They are now scratching their heads!!. I have only experienced this once before, when a different garage informed me it must be a short. Dont think it was, hence the new engine!!
Anyone got any suggestions?
Oh, and yes i have checked and the car has got oil it it!!!!
Thanks

Well if the switch is definatly good, and the wiring is ok, then it MUST be a oil problem.

Check the switch again, and see if there is oil on the wiring on the outside of it, this can cause problems sometimes.

Then you are going to have to start thinking about things in the oil that are playing with the switch (had a piece of swarf once and it put the light on solid) or blocking the oil lines. Or the pump is not working properly.

When was the oil and filter last changed? If the filter is badly clogged it will be reducing pressure.

You could do an oil change for the sake of a gallon of oil...