I've fitted an AFR gauge to my Escort Cosworth.
In idle it constantly oscillates between the values 14.7 and 15.6. This is without me touching the throttle and at (completely) steady rpm.

The same happens in every other driving situation with slightly different values except at full throttle. At full throttle the gauge is steady.

I've checked the gauge and sensor, petrol pressure, original lambda sensor, mass air flow sensor. I've even replaced all the hoses, changed the fuel nozzles and checked everything for leaks.

Does anybody know what the problem could be?

Thanks