Someone up there mentioned tapes sticking - true, some you can take apart and rehouse, but if the problem is that they are sheding gunk (rather than just too tight) that wont help. On the other hand you can bake them - not in a normal oven, the temperature setting isnt sensitive enough, but a food dehydrator will do it. Ive got one from a website dedicated to mushrooms! Put them for several hours at warm, and that helps - doesnt totally get rid of teh gunk, but it makes it a lot better.
if you put in tape baking to google you'll find instructions - mostly they are designed for reel to reels, but it works for cassettes too.