I have the same set but it always ran rough and jerky. 
I was about to sell it on Craigslist when I found this forum.
You can order a new Chassis for $40 or an Annie Chassis for $45 with metal drivers (looks better in my opinion).
Any of the 4 6 0 chassis will fit.  I ordered the 81097.  The new chassis run like a dream.

1. In terms of fixing your existing engine, First things to check is your power supply.  It should put out somewhere around 20 volts DC at full throttle. 
2. It could be that things are frozen up in your gear train which would mean opening it up and lubricating everything.  Also check to make sure there are no broken wires. 

There are a lot of posts on the forum about cleaning up Big Haulers.  Actually pretty easy to work on. 
A suggestion.   Grab some post it notes and scotch tape.  When you remove screws,  tape them to a post it and label where they came from. Makes it a lot easier going back together.   Near the top of the Large Discussion is a detailed description on repair tips.

I received my 3 Annie Chassis.  Boy do they run a lot smoother than my originals. 
Only issue was that the 81097 Chassis arrived with wobbly center drivers and I could hear something rattling inside.  Since it was the first one I was changing, I didn't realize it until I had mounted the old shell on the new chassis.   I opened it up and there is a plastic joiner that connects the two wheel shafts and it was in 2 pieces.
I  sent an email and pictures to Bachmann over the weekend and hope to hear they will send me a replacement.
I tried calling this morning but the phone tree choices didn't match with my problem.
I guess I just have to wait until they get back to me. 

Thanks for the posts Bill,
I have 3 older Big Haulers, Rocky Man 90015, Royal Blue 90016, and Emett Kelly 90020.  All run very rough and always have. I was getting ready to part with them and found the forum.  Ordered three different Annie Chassis today and looking forward to the upgrades.   
Thanks