I would think its definitely pickups causing it. I have this problem with N gauge stuff all the time - they are particularly prone to collecting fluff and dirt. You should be able to work out which pick ups are causing it by running the loco over a series of points - when it stops, check to see which pickup(s) are being relied on at that moment in time - that will be the one(s) causing it.
Other than that it could be a poor connection within the loco, but I reckon its most likely to be the pickups.
Anything else like track issues would cause more or less all locos to act the same way, so it sounds like a loco specific, i.e. pickup issue