It could be further down the circuit, but it should be related to one of the DRUM UNIT ZERO switches on that reel (i.e., a wire that is directly attached to the DRUM UNIT ZERO switch on the non-resetting reel if the switch itself is making good contact). As shown in HowardR's schematic, the RESET RELAY stays engaged until all eight DRUM UNIT ZERO switches open (each reel should have two of these switches--the ones shown with Y and G-B colored wires). One of these DRUM UNIT ZERO switches on the reel that is not intermittently reseting is not completing the circuit to the RESET RELAY (either because the switch is not making good contact or because there's a break in a wire leading to or from the switch in question).

The reel resets properly when it is at a number greater than five because all reels will receive five pulses during a reset and will continue to pulse up until they reach zero (and open another DRUM UNIT ZERO switch that controls this). When your problem reel is at a number less than five and it only takes five pulses to reset the other reels, then this reel won't completely reset, because the circuit to the RESET RELAY opens when these other reels reach zero (and the bad switch/wires on the problem reel are not completing this circuit).

Based on the description, it sound like the switches are working properly and so it sounds like the problem is with one of the wires on the DRUM UNIT ZERO switch in the part of the schematic shown by HowardR (with Y and G-B colored wires). As HowardR suggests, using jumper wires could help find the problem.

Good luck!

I had a similar problem with mine and it was the switch never closing. Another pinsider had a similar problem recently and it may have been a problem with a poor wire connection. Check out the following:

https://pinside.com/pinball/forum/topic/aladdins-castle-100s-score-reel-problem