Sounds like an escapement issue, and it's possible that just giving it a good cleaning will sort it out. You'll need some mineral spirits and a supply of cotton swabs (Q-Tips). You didn't mention which generation Sterling you own, so it's difficult to get more specific other than you can get at parts of the escapement by moving the carriage to its extreme left and right positions, and by turning the machine upside down. It's located dead center of the machine under the carriage. Gently clean everything you see in that are with mineral spirit soaked cotton swabs until they remain clean, and then try using the typewriter again.

Ah, so it's an end-of-the-line Sterling that hung in there for special orders while the newer 6-series models were already being sold. I have an early '60s Clipper that's a carry-over model too.