Land Rover Repair: Discovery problem, vehicle speed sensor, transfer case

Question
I have a 96 discovery. This rover has new coil packs, plugs and wires. Sometimes when you drive it after about 40min of driving it starts to fight its self.It almost acts like its hitting a limiter of some type. It slows down and go's normal and then slows down agian. At most for ten miles and then it's fine for a couple of hours. It does this maybe two or three times a week. The last time this happened I was going about 70 and when it happened the speedo droped to about 30 for just a few seconds and then went back to normal. Any ideas?
Thanks
Patrick

Answer
Hearing your description I'd suggest you check the vehicle speed sensor - located on the left side of the transfer case.  If it sends bad signals to the engine control unit the motor will cut out because the computer think's you're going too fast all of a sudden.