2004 GMC Yukon charging amp meter

My 2004 yukon recently began showning intermitent occasional hi amp charging on the stock amp meter. It started on a long, cold drive at high altitude. The amp gauge needle was 'pegged' for a while. I turned off all elec. Items I could afford at the time. The next day and for quite some time. No problem. Recently, about 6 weeks since that episode, it exibited the same conditions, at altitude, in fairly cold weather conditions. The battery checks out good, the alt/reg checks out good.