Oil is meant to be done frequentily, and most cabinets took oil. Grease lasts longer, but with grease you might need to disassemble parts to get to them. Old, dried up oil and grease can harden and act like a glue, but with persistance ns new fresh oil it will dissolve. Cleaning, oiling, and inspecting all moving parts should reveal the problem area. As I'm sure you know well, the machine head itself will cause the whole thing to hault too, it often helps to give an old treadle a complete over haul now and then.