Update 2 - I have followed the good advice above, and worked through several possibilities as to why I'm still seeing the locos stalling. I've made sure the pick ups are properly in contact with all the wheels in relation to axle side to side movement (thanks Butler-Henderson), and while I was at it, I cleaned the treads, flanges etc to ensure good electrical contact. I'm still not sure if I'm getting a momentary short somehow but the DCC controller does not trip, and I checked to ensure the wheel treads are not shorting at the converging of rails at the frog. Overall, the situation has improved with complete stops being reduced, but I'm still seeing the locos regularly 'stuttering' through the same points each time (although not all interestingly). However when the locos do completely stop it inevitably is when the central coupled wheel is over the plastic of the frog. When this happens, to get the loco moving again it either needs a slight downward pressure on the cab, or funnel, or slight sideways movement across the wheel axles. Of course, there is no such problem if the loco speed is increased.
I'm coming to the conclusion that I need to replace one of the points for an elecrofrog with associated IRJ to prove this cures the issue once and for all, then replace all the others likewise.
