Sounds to me like your batteries are NOT the problem. The volts will only drop a little with an unloaded test like running with the wheels in the air so that doesn't tell much, if anything. How steep are these hills and what kind of weight are you carrying? Most 36 volt carts I've been around only run 11-12 MPH to start with and drop to maybe half that or a little less going up a fair hill. Any idea what year the cart is. Some pictures of the batteries and motor bay might help a little. If you've checked the other things I suggested I'd check and CLEAN each and every major cable connection before I went further. Major cables are any that are larger than 12 gauge. Between the batteries themselves and every large wire / cable it has. I recently had one that was starting to fray a little and knew I need to put a new end on but let it slide awhile let me tell you it was causing a MAJOR loss of power and making a multi meter read incorrectly. There was a world of difference when I fixed that one slightly rough looking cable end! Most golf carts came with 4ga wiring for the motor & controls. That is often right on the edge of being large enough to handle the NORMAL golf course lawns. So if anyone has swapped a smaller cable in somewhere that could also be an issue. Most guys who ask their carts to perform better than stock change the cables to 2ga to let the power flow. Good LUCK!