The bike is a 2013 C5 500cc EFI(not G5...sorry I misled on my first post). It has about 10,000 KM (6200 Mi.). The problem has been pretty much the same for the last 3,000 KM (4 months). I live in Kathmandu, Nepal at an elevation of about 4500ft. The primary plug I use is the NGK BPR6ES and secondary is still factory Bosch UR6DC. A friend brought the NGK from the States because they are not sold here. Is there an NGK cross for the UR6DC? I wish I could get the Chevron Techron here, but I have been using the Gunk fuel injector cleaner the last couple of tanks. I have cleaned the throttle body. Fuel quality is, as you can imagine in a 3rd world country, pretty bad. The plugs are covered with black carbon build up, but that seems to be a normal problem with the EFIs here and in India. I put new plugs in, but it had no effect. Cold/hot ambient and engine temps do not affect the problem. It stumbles just above idle only. I know the fuel tank scenario sounds irrelevant, but the problem does seem to disappear with a full tank of gas and reappear when it drops below a half tank. The only other thing I have found is a broken vacuum line that runs from the filter box to what looks like a manifold block beside the battery. I'm not sure how to check for a possible fault code, but the engine light's function is normal. I haven't checked any wiring yet. I guess that will be my next step. Thanks for you input guys. Keep it coming!