Have you tried adjusting the heads on the VCR units you have? It's pretty easy to do, catch a quick YouTube of it. Just requires removing the chassis and a flathead screw driver.
I'd also suggest trying to rwind the tape by hand if a particular part of it is "bad"and try playing it from the start.
Last ditch effort, cut the tape to get rid of the bad section and splice the good sections together with a small dab of super glue. It binge within seconds and it's playable after a minute or two. I've recently done this for two tapes of my own with advice of a colleague who's also done it and it works flawlessly.