I've had the same bkay, I once had a 99 with a stuck stitch length mechanism, I could turn the adjustment screw in and out but nothing happened. It was a bit like your case, oiling every part over and over not really being abe to identify where the trouble lie. Left over night it was all freed up. As you well know, a 99 is much simpler than a 401 but still with enough links, knobs and hinges to make it difficult.
You are very lucky with access to nice and intersting models; 503, 601,... I was promised a nice and early 99 last week, but someone else ended up with it. Not a lot of machines turning up at the moment :- )