I have new transcontinental train set. I changed the addresses and both engines run slowly, butI have no speed control. What am I doing wrong?

I have an Overland Limited train set. In the past 18 months I have sent it in twice for repair.  It runs OK for about 4 months, then slows down to a crawl. the track is level, clean and I only pull the cars that came with the set.  I'm getting tired of sending $25 in each time and have it returned to only run a few months.  What can I do?
I also sent in a Pegasus recently to get a rod attached to the drive wheel.  It, too, has slowed down to a crawl.  What do you suggest I do with it?
Thirdly, I have an Emily HO gauge engine that will not pull the two coaches that come with the set.  I've added some weight to the cabin of the engine, but the wheels spin, but will not pull the coaches.  What is your suggestion about this?

I have a Bachman HO Pegasus.  It runs fine in reverse, but sporatic in forward. I've cleaned the wheels and track and oiled the engine. Do I need to sent it in for repair?  Also, I have a HO Emily engine.  It will pull the tender, plus one car, but when I attach the second coach the drive wheel spins, but has little forward movement. I've tried adding weight to the engine, but it still will not pull the two coaches.

I m sending in an engine for service.  How do I pay the service charge?  Do you e-mail me with this charge once you receive the engine?

I have a grandson, age 5, that is into trains.  We were recently given a non-working Big Hauler, Blue Comet.  He would be thrilled to just push it around the track.  I've heard that it is possible to disconnect the engine from the wheels.  Is that something that a novice, like me, could do, or do I need to find a person that does G scale train repair?  Thanks!