99' Nomad low power at high speeds

[raymatt]

I have a 1999 Nomad 1500. I was in the mountains last month on a ride and noticed the bike did not have normal power especially when climbing hills. With full throttle, it would bottom out. The guys behind me wanted to push me up the hill! I put in new plugs, air filter and fuel filter. I went on a ride this past weekend and had the same issue only I was on the eastern plains .... only small hills but with a strong head wind. It had trouble staying at 65 mph. I am at a loss. Any ideas as to what may be causing this?