Hunting of the engine

[natarajk]

Hello geeks,

I bought a 1990 525i.I can see the engine is running 700 rpm in idling(plus or minus 20rpm) and once in a one minute or two mintue the rpm goes down to 500rpm and coming back to 700rpm.This happens in normal idling without Air conditioner and also with air conditioner.My BMW never stalled till now.

is it normal or any tuning is required?
We need your advices.

thanks in advance.
raj