If it's "warbly", that sounds like either damaged tape or capstan/pinch roller tension.
It's this unit, right?
I don't know if you can access the pinch roller while this is in operation but if you can apply some pressure to the shaft underneath the roller, this would tell you whether or not it's a tension problem or not. I'd be glad to look at this with you. Haven't worked on anything like this in years.