Colin - it is possible that you are somehow emptying the sump, especially if you have a HV oil pump fitted. All the engine oil could be sitting up in the heads, oil pump sucking on air.
This is mainly a Ford BB isue, but HV oil pumps can do this to other engines as well.
Lastly- are you sure that you do have enough oil in the sump anyway? How did you decide where the "full" mark on the dipstick would be?
And even more lastly - too much oil can have a similar effect - crank runs through the oil, foams it, result = loss of pressure.
Have fun sorting this out now.
