High Voltage drop to slide motor

Before such heroics, I would suggest doing some probing around to see where the voltage drop is occurring if you can. It's probably easiest with two people. Put your multimeter on the lowest DC voltage range you can, and measure across or between various connections on the positive and ground paths while running the motor. As a start, if you can, measure the drop along the positive path (the battery positive terminal to the motor positive terminal) with the motor running, and compare that to the same on the negative side: are they about equal? If one is 1.5V and the other 0.5V, for instance, you probably have a poor connection somewhere along the 1.5V side. Checking various points along that side should show where it lies.

This likely isn't your problem, but I used to have a house battery (since replaced) that had a poor connection with unacceptably high resistance in the battery terminal between the battery post terminal and the threaded stud terminal.