ZF BW 255 No Oil Pressure

I have a ZF BW 255 gear mated to a DD 16V92.
I usually do not have oil pressure.
But when I do, I'll guess once every 50 starts, I get good oil pressure and I can go in and out of gear until I shut the engine down.
I was told that there could be trash in the control head or oil pump (pressure relief valve). I have removed and cleaned both and still have the problem.
Anybody have any suggestions.
This is on a 1987 80' San Lorenzo.
Thanks,
Dan

Thanks for your reply,
Every one in fifty starts I have oil pressure. When I have oil pressure the gear works normally.
These are analog pressure gauges which appear to be working.
When the gauge indicates zero oil pressure the gear won't engage.
When it does indicate oil pressure, about 180 psi the gear engages and I can shift forward, neutral and reverse, all day long until I shut down the engine.
Any thoughts?
Thanks,
Dan

Thanks for your replies.
I'm away now but will be back on the boat tomorrow.
I will pull the filter and clean again, that makes sense if there is trash clogging an oil passage.
The problem is recent.
Oil level is good.
Not sure about trolling valves ?
All mechanical.
Someone suggested that the pressure relief valve may be stuck in the open position.
Does anyone know where this valve would be located? Supposedly there is a valve set to open and dump oil back into the sump if the pressure exceeds 300 psi
I have removed and had cleaned the control head and oil pumps.
Rcrapps, do you know where the valve manifold is located to see if there is a broken spring?
Also someone suggested there may be a leak in the oil pickup preventing the pump from developing prime. Does this make sense?
And, is it correct that if I crack the oil line coming from the pump I should have oil spewing out?
Thanks again.
Dan