Recurring mis-fire issues

This is really a bit of an on-going saga.............basically I have had mis-fire issues with the 8 for the last 3 months, with the car going back and forth from the garage 4 times now.

The symtoms are the engine management light comes on, flashing at first and then after about 20mins of driving stays lit. The engine runs very lumpy, particularly at idle and fuel consumption drops to single digits. Each time I have had the problem its gone into the garage a been diagnosed as a failed coil. The sorry sequence of events over the last 12 weeks is as follows;

1st trip to garage - cyclinders 2 & 4 coil replaced. This fixed the problem for 2 weeks then the misfire came back; 2nd trip to garage - cyclinder 1 coil replaced. Again problem solved, this time for 4 weeks then same thing again with 3rd trip cyclinders 3 & 5 going. Again 2 weeks later and it had to go back again. This time, it was cyclinder 7 causing the problem, but asked them to replace 6 & 8 and do all the plugs as well (as I was told once one coil goes the other tend to follow, and thought it couldn't hurt to change the plugs as I don't know when they were last done).

..............so another 4 weeks later and the problem is back again.

I have searched this forum for related threads as well as trawlled the internet to see if there are any other causes, I read something about incorrectly fitted timing belts sometimes causing misfires but this could be a red herring as it didn't mention anything about it knocking out the coils. The first job I had done was the timing belt replaced when I bought it 6 months back, the garage I used were basically idiots so wonder if it could be related.

I would be grateful if anyone can shed any light on whats possibly going on, or even had a similar problem themselves?

Thanks in advance for any help!