Low oil pressure - light coming on sometimes

Hi I have the 4.0 thor engine with erratic oil light. Have changed the pressure switch ok for 1 day the came back on again. Now had pressure test at 1.7 bar.
Is it possible to change the relief valve and if so can it be done easily?
Is oil pump easy - ish to change & are they silly money???

If you know the answers to any of the above or can help in the way of advice - I'd love to hear from you.....