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As far as I’m aware of, stabilisers works by replacing the vertical blanking interval lines with clean black lines. When VCRs record, their automatic gain control is usually affected by the vertical blanking lines, which results in an unwatchable picture varying in brightness if there are spikes in the vertical blanking of the video source, whether from a poor quality tape or those that have been added deliberately as a copy protection measure.
Unfortunately, a video stabiliser will do very little to improve the picture itself. Generally their aim is to let you make a recording with a similar picture quality to what you see on the TV. This means that if your source tapes have colour fading in/out when played through your TV, then the recordings will also have this, even if passed through the stabiliser.
If you can get hold of a cleaning VHS tape, this is worth a try in the VCR your are playing from. Alternatively if you can get a lend of a VCR (even if a very old one), it would be worth trying one of your tapes in it to see if it gives a better picture. To give an example, my VCRs will play most recently recorded tapes fine. However, if I try to play old recordings (from ~15 years ago), the picture displays quite well from our old 1980’s VCR, but very poor (if not unwatchable) on our more recent VCR!