Problem With 2000 Fiesta Diesel Van

[nightflight]

have just bought an old 2000 fiesta diesel van - canny motor but with one problem. Sometimes you can be driving along and it loses power - it doesnt stall, just goes onto tickover and the accelerator has no effect. When this happens, the heater coil light flashes.Turn the ignition off and back on and it starts immediately - this can happen two or three times on the way to the shops just down the road, or it can do a hundred mile trip without it happening - anybody any ideas??