I had this very problem. After checking the driveline, rubber tires, and clean all traction surfaces with alchohol and q-tip (you mustn't over do it.) then all you have left is a weight issue. here's what I did. I replaced the tapered portion of the smokestack with a snazzy copper cone that I made on my lathe. You can use a drill and file if you hold it right. Then I removed the weight in the cab and replaced it with a block of lead cut out so that all possible nooks & crannies were filled. I used a milling machine but it could be carved if you wish. this helped considerably. I estimate a 2% grade increase.