Any open in the power circuit such as F & R contact tips, cables, terminals on motor. Regen contact has given the same problem if the contact does not pull in. Check coil resistance on Regen contact, should be approx 30 - 50 ohms. As you operate the truck the coil resistance may increase and cause the intermittent 29. Also I have seen the drive current sensor cause a 29 multiple times. Never found a bad speed sensor unless it was damaged. The logic does not see motor movement because of the open in circuit thus 29 comes up.