[bestgenman]

Do you know what the model number of the regulator is?  Voltage regulators started using a volts per hertz roll off feature some 25 years ago.  I know this sounds confusing but it is really simple.  As the engine speed decreases, the regulator’s output is reduced to the exciter which results in a reduced output voltage.  Depending on the regulator model, it can be fixed or adjustable.

At a first guess, it sounds as if the load on the engine is slowing it down and the voltage is dropping too low.  The kubota engine’s governor operates on a droop governing principle whereby the engine looses speed from a fixed position as load is applied.  Most all sets are configured this way.  There are some later models with isochronous governing (steady state speed regardless of load).  The droop is usually 3 to 5% but as the engine ages this can open up to much larger percentages.  What this means is, your engine’s speed may be low enough to trigger a fixed roll off setting in the regulator.  

Did the speed get checked when replacing the regulator, if v/hz is adjustable, did it get set to match the engine speed?

This is my best guess with the limited amount of information provided.