Have you solved this problem? Just about everything on the stacker needs to be working for 3rd speed to operate. All batteries and connects, key sw, door, sw, brake sw, SAS sw, mast sw, rabbit sw, interlock sw, etc. Voltage drop across the hole system is a big problem. I normally run a new feed to the B and C contactor coils. I also use a relay. The first thing I do is replace the interlock switch with a change over relay. Much cheaper than an interlock switch, it never needs adjusting and it also overcomes the voltage drop because it takes a lot less voltage to energize a relay than it does to energize a contactor coil.