1976 3ooD 115 engine running problems

[50cool50]

Hello folks i am back with the famed 76' that ran so well..
well up until 3 weeks ago when she lost power gradually and died on the freeway...

i got her home and took stock of my sherlock holmes hat and pipe

here's how she is behaving at present...

she starts up fine

she idles rough and i can hear a little tapping noise going on as she idles

the oil pressure drops after little while as she warms up
the oil pressure drops to around the lowest point on the guage then she idles rougher until she stalls... then she turns but wont start

i done a complete diesel purge and it didn't do much..

a buddy says it might be the shut off valve is stuck and is not allowing the ip to run properly?? or he says maybe the shut of lines have oil in

i tried changing the shut off valve. i got the top 10mm bolt out but the other 2 bolts holding it in look smooth and i am having trouble getting them out, anyone got any experience of changing one of these on a 115 diesel?
anyone got any ideas that i could try?

thanks for your replys