We had a situation in an MD80 on a short flight and my captain got a little slow at 36,500ft so called it out and he used alt hold and MCT to regain speed but it was too late. Using MCT actually lowered our N1 and we started to get buffeting so descended at 500 fpm to not stall. We were in the clouds and had opposite direction traffic at fl350 5 miles in front.
That is the only time in my career I Felt out of control as far as keeping our clearance. It never happened when I was captain even when my FO said we could cross a climb fix when transitioning to the 757. The 727was a slug so never accepted a clearance unless I knew We could do it.