XJS 4.0 Oil Pressure

[GriffinNo2]

My 4.0 XJS's oil pressure has always been at least 50psi at 1500rpm and above even after long runs on hot days. Recently, during long runs, if the egine is allowed to idle (eg at traffic lights) on resuming driving the oil pressure refuses to go above 25psi until over 3000 revs and falls back to 25 psi below 3000 rpm. Occasionally, if I switch off for about one minute then restart the engine the oil pressure goes back to 50psi at 1500 reve and above and behaves normally. This behaviour is intermittent, some days it never happens at all and on others it occurs immediately after the fast idle, once the engine is hot. Is there some form of pressure relief valve which may be sticking and causing this problem?

[nigel1]

With regard to oil pressure problems, the usual suspect is the oil pressure sender mounted on the LH side of the engine block.
The early type (which I prefer) is a pressure sender proper. Later, Jaguar introduced a sender which through some electronic trickery, would cause the gauge to read at 50%, no mater what the pressure (unless zero).
If you have the AJ6 engine, it will also be fitted with a seperate oil pressure warning lamp, which received a signal from a seperate sender.
I would replace the sender first, and see what happens, they are fairly cheap, about £25