no oil pressure!!

HI all got a good one for you all,
i have just converted my defender 2.5d to a 200tdi, but im having a night mare, finally got it going after it came with a siezed stater and lift pump, started up fine no smoke, tapped for a second or two till the oil got up there, switched it off went to restart and found it had no oil warning light so swapped it for the one on the old engine and worked fine restarted it and found the oil light stayed on idle so i adjusted the throttle and it went out, reved the engine and found it flickers on and off and then when it gets warm the light flickers on idle and then comes on solid until you rev it to about a third throttle, i have changed the oil to 10w30, and filter and have now just ordered a new oil pump,
is there anything else that can cause it???
sorry about the long essay lol

either you need a new switch or if pressure low its the bearings usually mains ,big and cam,not usually pump unless relief valve stuck open ,get pressure checked with guage

i have now, checked the dipstick tube it had slipped down and 2 and half inchs have now set it perfect and put the correct oil level in it, started it up light went out rev it up and nothing no light, i thought great fixed it left it ticking over for about ten minutes still fine drove up the road half mile and now its on again between tick over and thrid throttle,
its getting very depressing:doh:

i have tried 2 new switches now, no different