Bob – had this on my old van and it was a faulty oil switch, after testing the oil pressure to check. However, could be more serious like a lazy oil pump – Jim or Ken would be better advising on this. I have now fitted to my cars a mkII escort oil pump with screw on oil filter. Easier to change the filter without the oil running down your arm!!!
Oil loses viscosity – thickness, to most people – as it warms up so oil pressure on tick over falls. This is particularly so if the idle speed is set a little on the low side, while the setting at which the oil pressure switch operates varies within limits.
If you’ve got the oil nice and hot, the engine speed falls below normal on braking (which can happen) and the switch is set a little high, flickering will occur without any real problems. And if the light is merely flickering, that is enough on tick over; you’re still getting at least 5psi, which is enough off load at those rpm.