How long has it been since you've done a thorough clean and oil? These older mechanical machines are great, however, they do need periodic TLC to keep them running smoothly. You need to open the top, side and free arm. Slowly wiggle the hand wheel and move the selectors/dials, and use either liquid Tri-Flow Synthetic Lube or fresh good quality Bernina oil for CB hook machines. Also, look for the little holes that are oil ports. Stay away from the Basting Stitch mechanism located near the center of the needlebar!!! Do NOT OIL the nylon gears! Don't forget to turn the feed dog dial and oil those moving joints too. If anything is stubborn, a bit of direct heat from a handheld hairdryer will help it loosen.
The following is from a Bernina User Group (a Certified Bernina Technician):
Feel safe putting a drop of oil on any two metal parts that move together. Over oiling will just cause more debris to collect. Excessive oiling will cause failures. Here are some other tips concerning oiling.
Machines with basting mechanisms you just need to stay away from the middle of the needlebar.
First you must be willing to remove covers. And I refuse to suggest anyone removing the back covers of machines with electronics that have to be moved to get to certain oiling points. Especially without a anti-static mat and being grounded.
Use clean sewing machine oil free from resins and acids. Inferior oil may cause a machine to jam, due to the oil drying.
If a sewing machine is kept in the cold it should be left to warm an hour or so just to let the viscosity of the oil in the bearings to restore itself.
Motors are different and the bearings do need oil once a year or so but the motors changed in different machines. Some have obvious oiling holes to get to the bearings, some do not.
A drop of oil under the bobbincase on the hook race (the ledge the hook sits on) is the most often oiled spot. Techs say differently but I just say a drop every 4 hours of sewing or whenever you wind a new bobbin. Or just when you hear noise getting louder there.
As far as grease and gears go, grease is fine when you have metal to metal gears but the fiber gears don't need it since lubricant is built in. The gear grease traps wear particles and abrasive dust particles. Since the plastic is soft and easily abraded, grease would just entrap dust particles which will wear down the teeth, effectively destroying the gear.
Picture of oiling points for the stitch selector (but this is only one small area that needs lube!) See the metal gear just left of center, it has a little hole. That is an oil port. You will find many of them throughout the machine, but also hitting the moving joints will get those spots you've missed.
Below is a picture of oil spots:
Treat your machine to some much deserved TLC and she will return the favor ten fold!