We have a Singer 413 that had a sluggish motor that oscillated RPMs as it ran. Our LSMG told us to pull the belt off, and run it at full speed till it cleared up. He said it might need cleaned out. Then he said to clean them out he hosed the inside of the motor with lighter fluid then ran them. I did clean the motor but used break clean, it's a bit less flammable. The motor came back to life and it was running better and better ..... then it stripped the teeth off a plastic gear.
Like Charlee said, run it till it dies. These things get arthritis when they sit unused.
Joe