older spectrum diesels

[sd24b]

I have 2 8-40C's and a SD45 that run and track well until a train is coupled to them.  The GE's with 30 cars on a 2% grade drop the lead and center axle out of their slots.  The 45's leading truck jumps up and down.  Subsequently they derail.  Wheels are in gauge.  And none of my other 6 axle units do this.  Including those that are heavier or lighter.  Solutions or suggestions   Phil