You can get an idea about it in this thread:
But IMO first check for vac leaks going on anywere near that throttle body (ports on the intake, rubber boots, TB proper seating), then I'd check the valve clearances (you are into that anyways, aren't you?) And if everything seems to be fine check the TB sync.
If I recall correctly the rear flow sensor*? (is that what it's called?) is also hooked up to that intake (at the vac port), so it can measure intake air flow or whatever - so any leak in this system could also lead to misfiring/backfiring into the TB. So check the connecting hose for any cracks. Don't know how to check though whether the sensor is working correctly - maybe swap it with the front sensor, and see if the problem shifts to the front cylinder?
*Edit: Looked it up in the fische: 61041085100 PRESSURE SENSOR.
I belive the guys who adjust the TB with TuneECU, rely on the values of these sensors, so if you also have a plugin type flow meter / other syncing tool, you can quite rule out if the problem lays somewhere in here. Also to get the job done properly, if you are really willing to dig into the deep, finding out what's going on, adjusting the sync and the TPS, you'd probably need TuneECU (and lots of free time:)). That said, it can be anything different, like the TB not providing enough fuel/shutting off all the fuel on closed throttle because of a failing TPS, or misinformation from one of the sensors (o2, pressure).. I believe it's quite hard to diagnose, so if the simple things don't solve the problem, maybe living with it is easier and cheaper than fixing it trial-by-error.