(ET) Mower motor problems

[Stan Witherspoon <stwspoon speakeasy net>]

I have been having mower motor problems. I have replaced the bearings,  
replaced one brush housing, had the same magnet in two different  
motors come loose, and now the magnet in one of my my replacement  
motor housing has come loose as well.  This is in less than 12 months  
and less than 10 mowings. I can't seem to get through a single mowing  
without having to fix something.

I realize these tractors are pretty old, but I would expect that stuff  
that gets fixed would stay fixed.  Is there anything I need to watch  
out for on the mower motors? I have been through the HSM and all of  
the on line archives.  My mower deck has a loud rhythmic vibration  
that resonates every second or so that goes away when the motors are  
loaded down, like when mowing thick grass. I have balanced the blades,  
replaced the blades, balanced the replacements, disassembled, cleaned  
and reassembled the mower deck and replaced the wiring on the mower  
deck. I also have new fiber washers and I torqued the blades bolts  
according to the HSM specs.

Any Suggestions?

Frustrated,

Stan