Could be a couple of things.

The most obvious is dirty heads. Go and get yourself a head-cleaning tape, or have the lid off and give the heads the once-over with a cotton bud and an appropriate cleaner.

The other is maybe a little trickier as what you have described could be due to a head alignment or tracking problem. Tracking on older VCRs was adjusted manually which while awkward meant that you could compensate when the picture quality lapsed, or failed altogether. My mum has an old VCR, which while it still works has difficulty playing back anything which has been recorded on another machine. With manual tracking at her disposal the machine can be adjusted so that she can watch things which have been recorded elsewhere.

In more modern VCRs (anything newer than 5 years, I would guess) tracking is handled automatically by the machine and either the heads have gone west and it can't compensate adequately enough, or whatever circuitry governs this process has gone awry and is actually over-copmensating (not not compensating at all). I had a machine which fell foul of this. You could see when an 'alien' tape was inserted that the tracking function would try and adjust the picture, the picture would come, but the machine didn't realise this and kept on adjusting until it was gone again.

The decision then is to decide whether is worth fixing or whether a new one is required. If you know of a local, reputable, repairer they should be able to take a look and give you their verdict without it costing you too much (or anything at all).