If you are talking about sparking where the rollers hit the third rail, that is caused by the electric current jumping the gap between the roller and the third rail (arcing). Here are my thoughts:
1)If it is happening only on certain parts of the track, especially if it is tubular track, it could be there is a flat spot or depression in the third rail at those spot(s) where the roller "jumps" over the depression. Other engines may or may not have a problem, they all vary in how high they are off the rails, etc...but if they have it, too, in the same area, might be that.
2)I don't know what kind of track you have, but post war trains had pretty deep flanges. Some types of rail are more shallow that traditional O/O27 track, and it is possible that the flange is hitting the ties and "lifting it up" just enough to cause arcing. Not as likely a cause, but not knowing the track type it is possible this is a factor.
3)It could be for whatever reasons, on that particular engine the bottom of the power truck is higher off the track level then typical engines (it is riding higher/thinner, etc). That would even with the spring loaded rollers it is having trouble making contact. To fix this:
-One way, not sure I recommend this, would be to try and bend the arms that the roller is on down gentley, maybe use something as a lever to bend it down.
-Probably better way would be to take the rollers off, and put small washers between the roller assemble and the bottom of the power truck (fiber/plastic would be better). See if that makes it better, it may be you have to experiment with the depth of the washer to get it right.
