Part of the problem could be trying to climb cut instead of feeding the bit into the cut. I don't see where it would just stall and not cut but it could make it harder to cut and cause jumping or be hard to control.
It would help if we had pictures of the bit and the cuts that were made in the wood. One other thought would be if the Arms of the pantograph are binding and causing the hard to operate problems.
Your BRAIN Is The Most Important Power Tool In Your Shop. Turn It On Before You Turn On Any Other Power Tool.
A Disability Is Only A Disability If You Let It Be One