Hi,
while these may not be the cheapest encoders, they are rock solid and working without problems in my MB6582, since more than two years now :)
http://de.mouser.com...muFqx9z8g%3d%3d
I had some problems in the beginning after manually removing the "dent" where I accidentally bent the upper sliding contact too much upwards... which led to a bad contact between upper and the lower inner part of the encoder. But it was easily fixed by bending the sliding contact a bit down. . I´d also recommend that you "glue" the encoder case afterwards on the opening brackets with JB-Weld, this gives maximum strength - my unglued encoders were "wiggling" a bit after a year of heavy use :-)
So, if you are about to give up, I´d recommend you order one of these for testing, desolder your worst encoder and test with this Alpha encoder instead...
Many greets,
Peter