It doesn't sound like a common Nikon problem (had something sort of similar happen to an old Canon SLR once, long ago).
It does sound like something's out of alignment/needs adjustment/has a stripped gear tooth. An old camera mechanism has lots of clockwork elements to it to keep everything cooperating mechanically.
A quick look at the negatives will show if it's the fault of the camera or an automated photo processor.
Otherwise, the camera probably needs a regular maintenance checkup, usually called a CLA (clean, lube, adjust). This is usually between $150 and $175 and requires sending the camera away for a month or so. Nikon no longer services models of this vintage.
If you need something for photo studies, you can probably nab an old SLR off EBay for next to nothing, then send the S2 off for repairs. If the S2 is a family heirloom, it's worth getting it fixed.
Nikon S2, S3, S3-2000, SP, SP-2005 / Kiev 2a
Biogon 21/4.5; CV 21/4; CV 25/4; CV 85/3.5; the following Nikkors: 2.8cm/3.5; 3.5cm/1.8 (1956 and 2005 versions); 5cm/1.4; 8.5cm/2; 10.5cm/2.5; 13.5cm/3.5
Soviet lenses: Orion 28/6; Jupiter-12 35/2.8; Helios-103 50/1.8; Jupiter-8 50/2