NGX with F-18 performance

I had a weird one last night, climbed out of KABQ on the way to KDEN. Was on RNAV & VNAV fairly heavily loaded in the -800. Had a climb rate of +7000 fpm and when I reached cruising altitude of FL350 it overshot the altitude. Then after leveling off the throttles went to idle, I turned off AT but that didn't help as when I manually throttled up it would go to 95% and then go back to idle. Any ideas, Kyle ? It only happened that one time but is puzzling. I turned off Vatsim so they wouldn't freak out at the climb rate.