I have now re-laid my layout, which is now perfectly flat, running on a 4mm cork trackbed, and since relaying the only place I have had any trouble, both with derailments and locos stalling, is on a doubleslip that I purchased 2nd hand from ebay. Peco code 100, insulfrog.
The double slip is in good very condition and the rails are clean, but all the small locos regularly stall and my Bachmann Crab regularly derails whilst crossing it.
After reading this thread I decided to buy a new double slip (thanks John), and I fitted it this morning. The first thing I noticed is that the new double slip is different to the old. The point levers are in a different position (meant drilling new holes in the baseboards) and the rails have a slightly different configuration.
Now fitted I tried all my locos at various speeds across the double slip, not one loco stalled including Percy (0-4-0) and Thomas (0-6-0) and both Bachmann class 08s which would all stall on the old set. Not one derailment either, even when I sent the Crab over them at a scale speed of about 250 mph :o) .
If you have problems with a double slip, I would suggest you replace it with a modern rehashed version, as there is a definite improvement in the configuration of the current version.
Eddie.