Hi, Im looking for some advice please.

My 52 plate st170 has started stalling on me when slowing down to a stop. The problem first started a couple of weeks back when I noticed that the engine seemed to be misfiring when started up from cold. I changed the plugs and fuel filter which seemed to improve the situation but now things have taken a turn for the worse. Originally when I fired her up she would rev high for a few seconds and then would suddenly drop low resulting in the engine sounding like it was misfiring. The next time I started her up she would run smooth until sat in traffic and the revs would drop low again when idling. Now when accelerating I noticed that the engine was holding back and with very erratic performance. On my way home from work last night everytime I slowed down to a stop the battery light would come on and the engine would cut out completely leaving me with no brakes or steering which was a complete nightmare. I have done the self test diagnostics and no codes were logged.

Any suggestions would be greatfully received before taking it to the garage to get stung!