Like many others I have had problems with some of my 74490 motors that would become unresponsive after a while. I have tried some of the various suggestions available, but recently I stumbled across this link:

http://schnatterfleck.de...Endabschaltung_entfernen

As can be seen from the pictures, this solution completely removes the 2 micro-switches and replace them with permanent bridges between the contact points on the PCB.
I modified 5 problematic motors this way - took me less than half an hour - and the problems are gone! The important thing to remember is to set the switching time to the minimum (200ms on my CSII) in order not to burn the coils.

Your mileage may of course vary, but this worked for me.