940, Non-turbo, 120,000km

Ocassionally, I hear a pop sound, especially on load. There is no specific pattern, and it does not happen all the time. It comes on whenever it feels like it (Average once a day). It pop just once and then there is a surge like the transmission engages a higher gear. The car runs fine except that idling is a bit erratic, wavering between 750-900 rpm. During such shift in rpm the car shudders a bit.

I've done the following things but the "pop" is still there:

Change plugs (Volvo), and cables (OEM)

Injectors checked by disconnecting the connector one by one and noticing engine shaking quite a bit when disconnected from the connector. I assume this means injectors are okay.

rpm sensor and wires changed

throttle and IAC cleaned. TPS tested okay by turning the throttle slightly and getting the click from the TPS

metal surface of the distributors cap sand-papered, distributor rotor also cleaned and sand-papered.

timing belt quite new with only 25000km

The only other part I suspect is the oxygen sensor. Petrol consumption is noticeably higher and I've done a voltage test on the sensor. Can't say I've got the 0.2-0.7 V reading but I'm not good with the multimeter anyway so I'll have to rule the results out. (I'm getting a new sensor in a few weeks from a friend who purchased it cheaper overseas)

Can the O2 sensor cause backfiring?

Also the tailpipe is of a larger diameter than the original because the mufflers have been changed (from Volvo original to Walker). Other joints fits exactly as before. I don't know whether the backfiring started before or after I've changed the mufflers (center & rear) but I only noticed it some weeks after the mufflers have been changed. I read before that turbo mufflers can cause back pressure. Is it possible that the new mufflers are meant for the turbos? Can the turbos mufflers fit to a non-turbo without modifications? (Just a guess)

What else causes backfiring? Please help. I do not want to end up with something serious and expensive to deal with.

One more thing, petrol filter is 20000km old. Is this a possible cause for the erratic idling or backfiring?

Thanks once more!

Laozong