Caterham K series erratic oil pressure

[Sumpplug]

Hoping some wise heads can point me in the right direction. Recently bought a Caterham R300 with 1.8 K series. Oil pressure intermittently fell to zero on the test drive but the owner said it was the sender and he had a new one but hadn't fitted it yet. He gave me the sender which I fitted but when the engine is warm the oil pressure still read zero below about 1500RPM. Assumed it was the gauge and ordered a new one from Caterham but no difference. Convinced I was facing imminent engine failure I bought a test kit with a mechanical gauge. With this connected the pressure is 60PSI cold and 50PSI warmed up at idle and no fluctuations. So: its not the engine, the sender or the gauge, what on earth else could it be?

[Sumpplug]

Thanks both for suggestions, I'll check the common earth first which I think is under the scuttle somewhere if not that I'll try a new sender, then bin the lot and fit a mechanical gauge!

[Sumpplug]

Thanks, I tried tracing the earth from the gauge but it disappears into the loom. Having spent 10 minutes stuck in the birthing position with my head in the footwell and my legs over the roll bar I gave up. I'll try running a new earth to the gauge from a suitable place under the dash and if that doesn't work I think I'll follow the consensus and convert to a mechanical gauge

[Sumpplug]

Following my original post I am now in a position to give the definitive answer on how to solve the problem which is as follows.
1. Change pressure sender for one supplied with car, realise this hasn't solved the problem.
2. Replace oil pressure gauge £105 from Caterham. Realise this hasn't solved the problem.
3. Discover replacement sender is leaking oil. Overtighten until the thread strips. Order new oil pressure sender £93 from Caterham
4. Overtighten new sender until oil filter casing shatters into two pieces and oil drains all over garage floor. Realise you have made things worse.
5. Purchase new oil pressure adapter £36.00 from Rimmer Bros.
5. Fit new sender unit and rejoice that unexpectedly you've fixed the low oil pressure reading as the supposedly new sender unit supplied with the car had an identical fault to the original once.
6. Discover replacement sender is now leaking as you damaged the crush washer by previous over tightening.

I hope this helps anyone with the same problem. You may want to omit some of the steps above and save yourself a load of time and money.