Bachmann GP40 Problem

[LiveSteamer]

Ok guys and gals, I acquired a Bachmann GP40 from my girlfriend which was part of her father's collection before he passed away 20+ years ago. The locomotive was running not so great when I got it and now it is running even worse. I've checked all the wires and they are all hooked up properly to where they should be. There is the weights that Bachmann had put in there and I know if I take any out it will not have the guts to pull any of the cars of I have. Could it be a simple fix as to adding more weight on the chassis or would I just have to simply replace the motor in it? If that would be the case then I can have it fixed tomorrow sometime. I mean the engine couldn't even pull 6 coal cars and a caboose. It got to the point to where it wouldn't even pull just the caboose but then once it sat for a few moments it would pull the coal cars and caboose then right back to only being able to pull the caboose.

It could be the fact that the engine is probably well over 30 years old and it could just be wearing out and need a new motor. I don't know, I just don't want to bench this engine as I know how much it means to my girlfriend and I promised I would get the engine up and running and it was then this happened. Murphy's Law at best I guess. Any help would be appreciated.