My 01 1150GS recently has started pinging and losing power in 6th gear. It first happened last weekend on a highway run on I17 heading south from Flagstaff, AZ it was running fine then on a long uphill doing about 75mph in 6th it suddenley starting pinging and losing power. I had to shift down to 5th and the power loss continued. At one point my top speed was about 50mph. After I exited the highway it seemed to run fine on the back roads between 1st and 4th gears. The next day I rode some back roads on my way home and everything seemed fine. Then I got on the highway and for the first 10 miles or so the bike ran fine, then the pinging and loss of power started again. My first thought was maybe bad gas, so I filled up at another station and added octane boost. This did not help. I brought it to my local shop this week. They checked the valves and he told me they were really tight. So they adjusted them and said that was probably the problem. I picked up the bike and started riding home on the highway, the first 5 miles or so miles the bike felt fine then the pinging and loss of power started again in 6th gear. I turned around and brought it back to the shop. Thats where it sits now. I'm sure the valves were tight if they say so but I was also pretty sure this was not the problem. It seems to me something with the air/fuel mixture. I'm sure it could be many things but I'm thinking o2 sensor? Any suggestions? The worst part is when I got back to the shop the mechanic took it for a test ride. I told him he had to go on the highway for for an extended period in 6th to replicate the problem. He was back after five minutes, said he got it up to 110mph and noticed the left cylinder cutting out at 110. WTF?