Before this long thread heads off the tracks into talk of "trolling" we should all take a breather and recognize
a component that hasn't been mentioned yet but could be responsible for the OP's well-described original symptoms.
A few posts back in this discussion, pictures were posted showing a typical direct lever-cotrolled HST unit as used on B-series tractors. But that type HST is not what the L3130 has. The smaller tractors (B's) use this design, but the L3130 uses the next bigger size HST. Because the engine is bigger, speed can't be controlled with just a foot pedal and lever. These larger HST's have a servo-mechanism (regulating valve and hydraulic cylinder) between the foot pedal and swashplate. The servo-mechanism function is to adjust the swashplate hydraulically in response to the foot pedal position. It interprets pedal position as the "desired" speed, and applies hydraulic force to move the swashplate to the correct angle for that speed.
It is possible for the regulator arm to wear, allowing control of the swashplate to become too loose. Its possible for the "setpoint" to drift, allowing the system to call for more speed than was intended - thus loading the engine and bogging it down when the pedal is pressed a small amount.
This might help explain several things cited so far in this thread:
1. it would ONLY affect the wheel-travel speed (nothing to do with pto)
2. it would be most noticeable in H - less in M, even less in L
3. being a condition arising from part wear, it would become gradually worse over time
4. it would only be partly affected by oil viscosity - (type and temperature)
The machine should be checked out by an experienced HST person, someone who knows the several HST generations - this version is the next after what my L48 has in it .
The regulator can be removed and inspected without splitting the tractor (trans fluid needs to be drained). Someone on TBN posted a few years back about a severely worn HST regulator arm - with pictures - I'll try to find the link. EDIT:
here's the link
As always, prompt good-natured refund if theory is bogus. Dick B.