'Remastering' of an old cassette recording, But what is the distortion effect called? Options

[gutted]

I have several old recordings - originally from cassette - that I'd like to try and improve. Or 'remaster'

Anyway - there's one particular distortion effect that I want to try and correct, but I'm not entirely sure what it's called, so unsure which tools to search for. I think it might be wow & flutter, but the research I've done so far would suggest that wow & flutter involves periodic change in pitch of the audio (due to periodic variations in speed of the cassette).

The effect that I want to correct for is certainly periodic and obviously has a frequency, but it doesn't sound like the music is speeding/slowing. Instead, the music sounds like it's periodically getting slightly muffled then slightly clearer, slightly muffled then slightly clearer, slightly muffled then slightly clearer. Perhaps this actually is wow & flutter (?)

Can anyone help me identify this distortion so I know what tools I'm looking for to see if I can make it any better.

I currently use Audacity, so any sort of plugin for that would be interesting. Of if anyone can suggest other tools that I may be able to download/buy for the purpose, that would also be of interest.

Cheers!
Dan.