Are both rollers making contact at all times? Perhaps at the switch they are both for a short period off contact that is only manifesting itself as an issue at slower speeds. If your lucky, that will be your problem. My issue is with my Fast Track track that allows the unhooking and such. On my vintage engines I get the same problem you have, engine goes over it and it switches the e unit to neutral and I know it is making contact. So yours could be too. Newer engines from the late 70's don't have that issue. So there could be some kind of issue with the e units thinking they are having the voltage shut off when they are not on these older units. Don't understand how it can happen but you and I seem to have a similar problem.