Need Help / carb prombles? Not sure

I bought this bike in March of this year and I live about 4500 ft. I ride this bike almost every day back and forth to work and never had any problems with it. but I went riding a couple of months ago in the mountain by my house and it jumps up to 9000 ft. When I was riding then, when I started get up there in 8000 ft roughly the bike start to run a little bad, so I finished my ride and got back down to 4500 ft and the bike start running good again. So I knew that I would have to change jets next time I went up there again. Well I never got around to change the jets and went back up there last week and the same thing started to happen again around 8000 ft like I knew it would but when I came back down in the valley the bike never started running better. So I thought maybe I fouled a plug. I went and replace the spark plug and it did not change a thing. So I check the air filter and it was really clean, So I thought maybe some crap might be in the carb. So I just got done cleaning it out, and found nothing in the jets/needles. so now I stumped. What the bike does is that it starts right up and idles fine but with you start climbing rmps the engine start to cut out like it is running out of fuel but If you just give it more throttle the bike reveds up really good, and when you are driving it around, and come to a stop the bike idles back down fine but when the light turns green and you start to take off the bike cuts out real bad for just a few seconds and speeds right up like there was nothing wrong with it. Has anyone had this same type of problem? And when I was in the carb I notice that I has a 126 jet in it, What would be my best size jet to run with and these different type of elevations .So if I could get some help on here I would really appreciate it. My thought now where maybe bad gas? Electric problem? Any info would really help me! Thanks alot for your time!