You might try using a double conversion receiver from Futaba. With all of that metal for the signal to bounce off of, your problem may be echos, espically if the problem always occurs in particuliar spots along the track. I don't know if Futaba makes a 40Mhz version of the Double conversion receiver but it's worth a try.
I know it's unsitely but try the receiver antenna fully extended and sticking straight up first. If the problem goes away, then it is an antenna length problem and spooling it may help. If not, it's echos and you need a better receiver.