@Darrell the nylon that Weaver used in the drivetrains of those locos was not fully "cured," and it tended to shrink over time, usually splitting on the metal shaft. You may have one or more of the following problems:
(1) The universal joint collar (sometimes called a "helmet") is split, and no longer has a good grip on the motor shaft.
(2) One or both of your chain sprockets is split, so the spacing between the teeth is no longer constant. The motor is straining to overcome the increased resistance, and this causes the main drive shaft collar to spin on the motor shaft.
(3) One or more of your axle gears is split, which may cause increased or uneven friction down in the truck. This causes inordinate resistance in the drivetrain, leading to slipping of the motor shaft.
All of the parts for this drivetrain are probably still available from P&D hobby shop. Even Weaver brought out an improved version of this drivetrain before they were done with it. The updated version had a flywheel on the motor, and white nylon sprockets that were less prone to cracking. The flywheel makes a huge difference, and is a really worthwhile upgrade no matter what. It can be a little tricky to press one on yourself.
Your options are to order a bunch of new parts from P&D, or find a slightly newer Weaver (the black box mk2 version of this drivetrain) as a parts donor. These remain the best-looking RS3's in 3-rail, and will run very smoothly if you have the patience to tweak them and sort them out. But they're not durable or great pullers, not a lot of room to add sound, etc. Since this was made, MTH, Atlas, and Lionel have also offered the Alco RS with speed control, sound, operating couplers, etc. Unless this loco has a ton of sentimental value, I might just save my money for a newer model. My $.02.
