HP-67 card reader and magnetic cards questions

[albertofenini]

Hi everyone,
We really need some advice from the experts as we are experiencing the following :
Some of the HP67 that we have restored over the time and have been tested at the time for card reading and writing
are now showing unreliable behavior when used after some storage time (few months ...)
Some have stopped to read at all,
others read and write randomly, it happens that the same card can be read four times and then at the
fifth try it can't be read anymore, then maybe half an hour later it can be read again
We have tested different standard packs with no improvement
What could it be ?
We have used rounded o'rings to rebuild the gummy wheel but this happens also on the units where the motor and shaft coupling has been redone
Also, when this happens we clean the cards with cotton and isopropyl alcohol, is this good or it may affect the cards ?

Thanks for sharing any idea useful for troubleshooting this, we would like to have them as reliable as possible

take care !!