V8 oil pressure (again)

I was having trouble with the oil pressure relief valve sticking on my V8 3.5 (from a Rover P6) fitted to my series 3 Land Rover.

I wasn't able to find anything wrong with valve or pump cover when I took it apart but the valve still occasionally got stuck.
I have now fitted a tadpole type valve which seems to work most of the time, that is its not sticking as before.
But when starting from cold the pressure at idle is 40psi and at higher revs will increase up to 60psi. Once the engine is warmed up idle is 15 to 20psi, between idle and 2500 rpm the pressure increases up to 50 to 60psi but just after 2500rpm the pressure suddenly drops to 40psi and stays there for higher revs.
This seems a bit erratic to me as the original valve (when working) was around 30psi at idle hot or cold and at around 40psi the rest of the time, but at least there is good pressure there.
This is working fine when driving around locally but sometimes when on a longer run (I have a 40mile drive to work straight along a duel carriageway) where the revs are staying at a more or less constant 3000rpm after around 5 mins the pressure gradually drops as low as 25psi, slowing down to around 2500 rpm the pressure rises to its normal 50 to 60psi. But speeding up again it drops to 25psi again.
I've tried letting it idle for a few moments and increasing the revs for a few moments but it still goes back to 25psi. I've also tried hitting the pump cover with a hammer.
I've noticed also that when this has happened the idle pressure at the end of my journey can get lower than 10psi.
This doesn't happen every time I do the same route to and from work but about half the time.
Driving around where I'm constantly slowing down and speeding up this doesn't happen its only when sitting at constant revs for a while that the problem starts.
The pump has been modified by having the longer type pump gears fitted and the pump cover is a flotec type for the remote oil filter.
The only difference between the covers (around the valve area) is that the distance from the valve seat to where the port opens is about 3mm shorter than the original type.
I don't think the modifications are causing the problem as it had worked fine for a few thousand miles previously and I've checked all the parts for damage and wear, all are OK.
Anyone got any ideas what is going on.
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