Has anyone had any issues in the past with the oil light coming on when the car is on idle?

Basically, my series 2 is fine when driving and on idle when the fans not running, but when the fan kicks in and the revs drop slightly the oil light comes on. Blip the throttle or drive away and it goes off again!?

I thought this might be the oil pick up pipe being blocked, a common issue, but even tho this has now been cleaned out, the issue is massively improved but it still happens

Have just bought engine flush, new oil and a new oil filter so will see if this makes a difference............. just had a kent new cam, lifters, springs, tappets etc etc all fitted too within the last 200 miles so hoping this isn't major!!!!!