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ok, so the good news is that you are using a system that has all the capability to allow you to collect the data that will hopefully let you identify possible causes of motor overheating. First step may be just verifying that, are they in fact reducing power due to temperature or are you seeing the effect of something else such as a depleted battery. The bad news is I can’t collect that data for you, that will be something you have to figure out how to program or use the other features (i.e. motor dashboard) that the V5 has to help you collect it.
6-8 pounds seems pretty light for a mecanum drive robot, perhaps post pictures for us. I would be very surprised if you could run it for 2 hours, unless it was sitting motionless for most of that time. This mecanum drive ( 20 motor pushbot ) when run with 4 drive motors usually runs out of battery power in 15-20 minutes if I’m driving it constantly.