The curves envolved are Hornby R607 double curves. The locomotives (Diesel) approaches the curve in question and negotiates at speed set by the controllers control knob. That is until the loco reaches the fourth curve, when the speed of the loco reduces to a slow crawl - picking up speed again on reaching the end of this curve and onto the straight rail. No alteration is made to the controllers variable control knob at anytime the loco is travelling on the layout.
Please, any suggestions. Reading voltage accross the two rails, shows no variation in voltage drop anywhere on the layout and that includes the curve in question.