VW Golf Exhaust Backfiring?

Hi all,

I have a 2000 VW Golf mk4, 1.4 petrol.

When I am driving, sometimes if I accelerate, it feels like the exhaust is building up, then a sudden burst of speed happens as if the exhaust has cleared and then a small back-firing sort of noise happens from the exhaust

The 02 oxygen sensor has been gone for a while, never bothered getting it fixed as they wanted 200 for the part + labour extra. I was told by a mechanic it wouldn't affect the running of the car, only maybe the NCT. It passed the NCT with the light on, so never got it fixed. Anyway sometimes if I have been on the motorway for a bit, slow down and accelerate, it will bac fire and the emissions light will turn off, flash a couple of times and then turn back on!

I only got my exhaust changed at the start of the year as the back box in the old one was gone.

Any ideas what this could be?

thanks

Any ideas what this could be?