The OEM throttles for Scalextric Digital are not particularly high quality, and the sliding resistor can get dirty or be inaccurate sometimes.
Pull the throttles apart that do this even after you've calibrated them. Pop off the bits that span across the two halves of the controller, and then remove the screws and it will come apart easily. Get a cotton swab and some electrical contact cleaner, or meths/isopropyl-alcohol/denatured-alcohol, and use one end dipped in cleaner to wipe the sliding resister down on both sides from end to end, then use the dry end of the swab to get up any left on cleaner. I've also been using INOX MX3 on mine, because it does such a great job of making that kind of contact work well.
Aside from that, there's a mod that can be done to them, where you solder a little jumper at one end of the slider across two of the contacts, which provides better grounding of the sliding resistor. This was invented and shown in detail by Rich G on SlotForum.