The left hydros tend to accumulate a lot more dirt than the right so overheat .
This causes the oil to thin & the hydro to loose power on the left side.
So clean the outside .
If the problem persists change the oil and go for a heavier oil.
If you go to the tranny factory web site you will see the parts break downs for your trans
You will see that the valve chest rotates on a plate with slots in it.
The slots allow the oil to enter & exit the pistons.
When the oil gets thin it get lazy and passes between the valve chest & the plate rather than going through the pistons.
So you loose power