I have the Rivarossi Cab-forward and they have essentially the same drive train. Make sure that your boiler weight/frame is not warped. It was in mine (hence the $25 price tag on it at a swap meet). By carefully breaking it at the warp points, then carefully regluing them using JB Weld, I was able to rescue the frame.
Next, when reassembled, check to make sure the gear towers are not binding at the thru points on the bottom half of the boiler shell. While all 8 drivers of my 4-8-8-2 were touching the ground, there was no flex in the drive train so that the drivers were not in contact with the track at all times, especially on hills. By filing around the holes slightly, I was able to free up the towers. Nothing busts your bubble more than having an articulated that can't pull 5 cars up a 1.5% grade.
With the top of the boiler off, you'll notice that the drive train between the two gear towers has a keyed plastic drive shaft. In each end there is an oblong hole that mates with the metal shaft coming out of the tower. Make sure it is not stripped or binding and that it is long enough. Mine was about a quarter inch short and so was not turning the forward set of drivers all the time---OR---binding up at other times by being sloppy enough to jam on its mating shaft. I cut mine in half and extended it with a quality sleeve made out of shrink tubing. Unorthidox yes, but even with the 3 and a half ounces of additional lead packed in the boiler, she still humps 25+ NMRA weighted cars up a 1.75% grade on the club layout.
Make sure all your drive rods are free and not binding, there may be a driver slightly out of quarter or a popped rivet in the linkages. Finally, clean and lightly lube it. This may sound like a lot of work but the cheese eating grin on my face during Run Night at the club marks how well worth it it was. My 35 year old Grand Dame pulls full freights with the best of them high dollar, blue blood Kato and Spectrum units. Here she is in McCue city yard sporting her super bright LED headlight.
Lat me know when you're ready for a decoder install and I'll guide you thru that, too. Post some pics, I just plain love Rivarossi's "Poor Man's Brass" and am currently working on a 4-6-4 Hudson as we speak. Blessings, BC