Lionel Union Pacific Dash 8-40C coupler issues

I recently purchased a Lionel Union Pacific Dash 8-40C at a swap meet.  These were produced in 1989.  The engine is in good shape, cleaned up well, and has two powered trucks.  When running the engine and pulling 10 freight cars, it pulls well, but becomes uncoupled after coming out of a curve.  An inspection of the rear coupler on the engine shows the coupler not to have become uncoupled after becoming loose from the consist being pulled. The lead coupler on the car directly behind the engine does not show it to have become uncoupled either.  I have tried putting three different boxcars directly behind it, and none of the lead couplers have become uncoupled. However, the same result!  It appears to be a load issue.  Reducing the number of cars being pulled to six takes care of the problem, as the consist never becomes uncoupled.  The couplers on both the engine and the lead car line up perfectly, as they are both the same height.

i have other engines that I can use to pull a longer train, so I am not limited to this engine.  Any suggestions on what other procedures to try to correct the problem?