I found out what the problem was with my 127. It was the original one piece stamped presser foot. It has short stubby toes and when it hit any raised area the toes would just but up against it and the machine would stop feeding.
I put in a later vintage hinged foot with longer more curved toes and it worked nicely. So that problem is fixed.
As for your tensioner, remove it from the machine, take it completely apart and clean it thoroughly. Make sure the disks are smooth and free of rust or any build up. Polish them if needed.
Reassemble it and adjust it and I'll wager it sews better.
Do the same for the shuttle. Remove that dinky little screw, pull off the spring and clean it inside and out. Reassemble and adjust and it will make a big difference.