Proportioning valve in manifold can intermittently cause problem. Might be sticking or debris in valve. When the lowering valve opens and the prop valve is not ramped to proper position you will have fast lowering even if you are choosing slow lowering.

Hmm, short in lower button circuit, short in wiring harness to activate load hold solenoid valve , I would go with the short in the wiring to switch or defective switch.
My 2cents worth!