Slipping magnetic cards

[AndiGer]

Dear Bob,

Thank you for your reply. That post I already had found (and practised but a little modified using paper that is as thick as the card).
My actual problem is that the cards (have several with the problem) run thru smoothly but when the card is nearly thru the motor accelates for a short time before it stops and card is thru - but resulting in an "Error" display.
Sounds as if the card slips a very small way. Other cards don't behave like this but the faulty card(s) in another reader behaves the same. So I know it should be a card problem. Seems like the card is slippery somehow. Already did some cleaning on the upper side of the cards using some alcohol.
Don't believe re-writing the card will solve the problem.