I want to add that some little parts of the tracks are the old ones..but most of them are the new ones.. but on the old onse the loco can run without problems (even if there is also a bit of rust on the rails (not the middle point contact)

Try running a track cleaning rubber along the rails where you get the problem. This usually solves this sort of problem for me.

If cleaning the track does not work maybe add a few additional wires to the track at the problem areas.

Metpo:

I don't know if Goo Gone is marketed yet outside the US but I'm sure any citrus based household cleaner will suffice. Here is a description of the product:

http://www.magicamerican.com/

Lighter fluid is the fuel you put in cigarette lighters.

I do the same to clean the rails as above.By hard running...do you mean the loco runs noisily over the ballast?

I have found that i sometimes get ballast onto the rail itself.Not the top of the rail but just enough at the bottom so the wheel flange catches it.I use a stick with sandpaper glued to it to clean it off.Then they are nice and quiet.

Henrik: All metals are polished by an abrasive. The rails that you are running on were polished at the factory that way. Your jewelry was polished with an abrasive compound in a jeweler's cloth. That is how metal gets it's shine...by using finer and finer grit abrasives.

The best method I've found for ballasting:

Lay the loose ballast - use your fingertips to shuffle it into place, making sure it doesn't jam points or come above the sleepers

Mix PVA glue, washing up liquid and a little water in an old jar

Use a syringe to pick up the glue mix, and very gently flood the ballast with it. You need to use a gentle trickle as any pressure will blast the ballast everywhere and make a mess. Leave to harden for a few hours (checking that points, etc still work) then give the rail tops and pickup studs a light rub with a cleaning block.