As described in this video, my tractor was exhibiting the following symptoms:

1. Power loss (especially traveling uphill)
2. Transmission laboring (noise, vibration) at idle
3. Rapid RPM drop on deceleration

I discovered the cause this morning. The implement (subsoiler/ripper) on the 3point hitch was in physical contact with the top loop of the FOPS bar, which I had down in the 'stowed' position. Since the hitch couldn't reach maximum raised height, it was bleeding off hydraulic pressure from the transmission, thus causing the symptoms above.

THANKFULLY I discovered this on my own, before paying a mechanic to troubleshoot it. Whew.

So, if YOU own a hydrostatic (HST) tractor ... from any manufacturer, and experience the symptoms of degraded transmission performance ('power loss') above, check your front and rear hydraulic systems to make sure they aren't bleeding hydraulic pressure from the transmission.