It's always bloody coils....
With every old GS that comes into my shop with running problems, I pretty much refuse to start pulling apart things that customers have read on forums until all four coils have been swapped out.
All four coils can be replaced for the same cost as having a tech scratch his head (and arse) for two hours looking at things which are highly unlikely to have failed.
76k miles original coils and on the 3rd fuel pump, I wish it was always coils as running an AFR of 18 to 1 is not healthy for the exhaust valves.