Kubota ZD331 - Diesel - Bogs down in LOW grass

Hi guys, 
new to the forum and fairly new to Kubota Zero Turns.
I have a 2012 ZD331 zero turn diesel
2nd season I've used this to cut about 5 acres of grass on a rural property.

Recently, I've had an issue where the engine seems to bog down at slow speeds like its in deep grass, when its not. It seems like it wants to stall but usually doesn't.
It happened 8x in a 4 hour cutting session over the w/e.

This situation seems to almost stall the engine, however it seems to be the blades that are slowing down NOT the engine (if that makes sense).
If I disengage the blades that helps it to recover, however it seems from what I can tell from the engine, when I disengage the blades the engine rpm are still high....meaning the engine rpm were not bogged down, they are still humming right along
It seems to be at fairly slow speeds when the drive handles are slightly pushed in the forward position.
Pushing the handles forward does NOT change the bog down situation much....very slight. i can usually recover the blade speed if I pull back on the handles and/or disengage the blades for a few moments. then when I reengage
the blades I'm good and cut again.

Its very intermittent. It happens going up a hill or down a hill or on flat ground. I can not reproduce the symptoms at any time...its random. I've changed the air filter and both fuel filters.
I'm stumped.
Could it have something to do with a belt driving the blades slipping? is there a way to tell if belt is going bad and may be the culprit?
Is there a sensor on the handles that could be problematic?

I haven't had much luck on the internet regarding this issue and a ZD331.

Any help or direction is much appreciate.
Thanks!
JR

Kubota ZD331 - Diesel - Bogs down in LOW grass