The problems keep on coming.
Today I wanted to measure the efficiency. I started the motor and let it run for a while till the bearings warm up. I have not run this motor for longer periods of time before. After 5-6 minutes I noticed, that the speed is slowly dropping and the current consumption is rising slowly. The motor became slower and slower till it reached about 1000RPM and I stopped the test. I noticed that the bearing holder on the commutator side has become hot, I could barely hold my finger on it. At the same time the other bearing holder was only moderately warm. This indicates that the heat might come from the friction in the bearings. When I started the motor some minutes later, it started with high RPMs but then started to slow down again. This is weird. Also I now have a bolt on each bearing holder so that I can tighten it so that the outer part of the bearing can not rotate inside of the bearing holder:



If I tighten it too much, the speed drops, if I tighten it not enough, the outer part of the bearing starts to rotate inside the bearing holder and the motor becomes very loud and amp draw increased.

So far the only solution I see is to replace the bearings with ones that can handle a little axis shift.
This is frustrating