Diode testing: rising voltage in reverse direction?
When testing parts in-circuit you have to be aware of the affects of other parts in the circuit that end up in parallel with the device under test. On some very sensitive circuits it's even possible that a multi-meter could cause damage though this is pretty rare.

I am almost certain that the slowly rising voltage is not indicative of a fault in the diode, but instead is the result of a capacitor in the circuit charging. Capacitors in power circuitry (and you probablly wouldn't be using a diode that big in a non-power circuit) can be pretty big and multi-meter diode test currents are pretty low, so it may take a while to charge to the voltage where the multi-meter declares it open-circuit (or where some other component in the circuit activates and stops the voltage rising further).