Hunting idle after replacement TB fitted.

Hey all.

Got a 1994 Pre-FL N-CVT K11.

I just had the TB replaced with one already refurbished, and I've run into a strange running issue. Coming to a slow from any speed such as 40mph, the CVT box disengages like normal under 15mph but the RPM will go (800-1500, 800-1500, 800-1500) before settling back to the normal idle (rpm is estimated as no tacho)

This makes moving off and slow driving around villages a bit troublesome.

I've given the injectors and airbox a spray of carb cleaner to try and find vacuum leaks but found nothing, and some older threads mentioned the lambda sensor could be at fault, it would be the original and the car was run before I took ownership in 2017 with a broken MAF for a unknown amount of mileage.