Has anyone thought of Brakes? It sounds like you have a variation of a cable elevator or in rolling terms a Transtacker such as Raymond Manufactures. I am going to go out on a limb and suggest it would be criminal for any engineer to design a piece of equipment that relied on a motor to hold position and not a brake system to hold the position. The first question should be what is the load and how is the mechanical system maintained. There are great benifits to using AC technology BUT I am not sure they are cost justifyable in revamping an older system.

Please go back to the basics, check the doccumentation. You need to find out what the engineers designed the piece of equipment to do then see what the service provider has bypassed and thirdly check to see what abuse, overloading or missuse the operators are adding to the situation.

A well maintained DC System should last for decades if: properly maintained, supplied the correct voltages ( dying batteries will kill electrical componants mostly Brushes and contactors) and operated within the design parameters.