Possible overheating coils?

[Kookas]

After riding the bike at 70mph+ (motorway speeds) for about 50 miles, the bike will stumble when it gets near 90 (start lurching and dropping speed). The further I ride, the lower this speed gets, until I'm eventually on the hard shoulder waiting for the old AA to pick me up (this did actually happen!). The spark plugs are both pretty much brand new (and this was a problem before I fitted them as well).

FWIW, the bike also idles at really low RPM, so whenever I stop I have to hold the throttle slightly to keep it from stalling. But other than these two problems, the bike rides great, doesn't feel like it's lacking power, can do 90mph+ (until after 50 miles or so), etc. It starts pretty much immediately without hesitation, although because of the low idle speed you do always need either a little choke or a little throttle.

Of course for the idle speed I might just be able to adjust the screw, but I haven't yet because I'm thinking that the idle speed might possibly be related to the other problem.

[ajensen]

Have you tried putting the petcock on prime when the engine starts losing power?

[sledge]

I agree.
On the face of it I would be more inclined to suspect the fuel valve rather than an electrical issue.

[Kookas]

IIRC I did try that when I broke down from this a few months back and and it didn't have any effect, unfortunately.

[RyeGSF13]

I'm having the exact some issue with my 04 GSF. I was out for a weekend motorcycle rally. the bike ran good the whole time, on my way home though it started to lurch forward and lose power. While on the side of the road I tried to start it a few times. It would start right up then when I got to 5th gear, same issue. This time it would not start at all. I gave it 20 mins while I called for help. After this period the bike did start  but just wanted to die like before. It's at the shop now.  Mechanic ordered a carb kit and I am awaiting news.

[Kookas]

Apart from electrical issues, it could also be rust from the fuel tank or similar apparently. One guy said the rust clogged the fuel lines more the longer he rode until it started to stall the bike. That said, I don't think my tank has a rust issue. I'm going to try installing new coils and if that doesn't work I'll look at either the carbs or the CDI next.