I have power to the train and it runs but it moves extremely slowly and sommetimes comes to a stop.

I'm assuming the train is track powered with a power pack, and not battery powered, so there's actually a few possible solutions. I'll work from the easiest/cheapest, to the more advanced/expensive.

First, especially if the track is old or been in storage a while, the track may have a thin film of dirt, oil, or rust that is insulating the wheels from picking up the power they need from the track. Try cleaning the track with a mild soap but BE SURE TO DRY IT COMPLETELY before reattaching the power. Also, clean the wheels but use a cotton swab and very little water. One drop inside the motor might destroy it from the inside. You can also try a special track cleaning solution sold at your local hobby shop that has HO model train sets. That will work better, especially on lightly rusted parts. Also, make sure all the track connections are tight and free from contaminants.

Then, if you're feeling brave, you can check the motor of the train itself. Disassemble the locomotive and find the motor inside the housing. Check the wires leading from the wheel area to the motor for any corrosion, or loose/broken wires. If it's not the wires which are easy to solder back in place, I wouldn't recommend going any deeper into the motor as it's better and less frustrating to just buy a whole new set.

Finally, the problem may lay in the power pack (inverter) itself. I personally don't bother trying to fix a power pack, but if you know your electronics you could run some tests of the output voltage to the track, and the voltage on the track at various points. Again, it's not worth the time or effort to mess with the power pack unless you know what you're doing. You could get seriously injured or killed by the voltage/amperage running through those things. They even have enough stored electricity after they're unplugged to cause some nasty electrical burns from the capacitors.

Hope this helps and you're able to fix the problem so you can get back to enjoying the train set. I've always enjoyed watching them for hours, especially when I set up my track around the Christmas tree every year! Good luck!