If the following rings any bells I would love to hear any suggestions:

It's the worst kind of problem ie intermittent... Basically the bike loses all power for a second or so and then picks up again exactly as if I let go of the throttle and then slammed it on again. It only happens at altitudes above 3000 metres and only after a solid hour or two of continuous riding. If I stop and turn the bike off for 10 minutes it usually goes away but sometimes not. At one point I thought it was the breather tube on the tank being blocked and so I pulled over when the problem was happening and opened the tank cap, thinking that if the breather was blocked, that would clear things up. One time this made no difference and the second time I tried it it killed the engine and I had a nervous few minutes in a desolate place here in the Peruvian Andes unable to start the engine I don't really understand why this would kill the engine but maybe the system is pressurised in a way I don't understand??

I have changed the spark plugs and looked at the injectors (look clean enough to me). I was wondering if it was dodgy fuel as can be common in the Andes but the last time it happened it was fuel from the coast and of a higher octane which is supposed to be much more reliable. At that point I cleaned the air filter but it didn't seem particularly dirty. It's possible that has fixed it as it hasn't happened since but I am dubious as the following day was just descent and I haven't been back up high since.

I will be heading to Bolivia which is all about altitude and lacking in much mechanical support soon and I would like to feel more confident that I won't get stranded somewhere.... I am in Lima for the next day or two which is the nearest thing to civilisation I will see for a while (although the BMW dealer here has not inspired my confidence by messing up a simple valve clearance check..) so here is the place to take any action...

Thanks for any suggestions!

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F650GS twin - Enfield Bullet 350 - Honda XL250 Degree