Hi Max
I'm not surprised about the chain problem as I had exactly the same with mine, I had to make an adjustment system to hold the motor unit in place with long threaded bolts as adjustable stops. You can see that in the pics I put up a couple of post back.
I'm not sure what you mean about the battery / power problem.
Just thinking out loud.
Assuming that the battery indicator is working OK, you shouldn't see a reduction in speed if it shows only one LED below full power. If it shows more or less full, then the batteries cannot be at fault!
Are you saying that it show this level when driven under power? or when idle?
The voltage will recover quite quickly when the motor is not running.
Do you have any lights fitted? If so the brightness is also a good indication of battery level.
If you have access to a voltmeter, have it connected across the battery supply and see if it indicates a change when the speed drops.
Its also worth checking to see if anything gets excessively hot. Electrical or mechanical.
If it runs for 40 mins without a problem, I would doubt that there is much wrong, except the batteries, but the level indicator seems to suggest otherwise.
Hope you can glean something useful from this
Best of luck
Walter