I've had many of these types of problems in older cassette decks. The problem is that the rubber on the idler wheels has hardened and is too smooth.
Assuming you can find new replacement idler tyres, rather use new.
Assuming not (my experience), first soften the rubber with an alcohol-soaked tissue held to the rubber wheels, while they are turning.
After the rubber softens (you will see that the tissue is not blackening from the rubber any more, take a Japanese knife, and hold the blade to the rubber (while turning) shaving off a miniscule layer of the old rubber.
Reapply alcohol.
You should see an improvement in the function of the deck.
Menahem
Learn from the mistakes of others - you'll not live long enough to make them all yourself!