How old are the batteries in the cart now? There should be a date code (letter and number) on them somewhere, usually stamped into the post area. Example, mine are stamped C-4 which indicates Mar, 2004.
37.9v is not ideal and shows about 90% charge and could be signs that 1 or more batteries are failing.
A mountain course is demanding on batteries so an upgrade to 4ga battery cables would help if it hasn't been done already.
If you change the cart from 36v to 48v you need to make sure the controller will handle the voltage. Get the specs off the controller and post the info for more help on this. You would also need a new 48v solenoid, diode and resistor, less than $100.
Lead in batteries = run time, so 6x8v is much preferable to 4x12v.
With a change to 48v you can expect an increase in torque and speed of about 30%.
Some days you're the pigeon, some days you're the statue, I'd rather be the pigeon!