Where to get a front pump for a 42RH (A500)

I would also check the transmission cooler,it can also cause problems if it doesn't flow or is restricted.
When the converter comes on it releases fluid from the back side of the converter to apply the clutch,so if the cooler is restricted or plugged it can hydro lock causing the the converter to apply with out any electronic input.

The coolers in radiator are very problematic, if there was any debris in the transmission now or before it usually collects in the radiator where it is not see until the fluid is to operating temp and starts to break loose causing all kinds of issues,and with a mechanical governor it starts in the wrong gear won't down shift ect..
When in doubt, bypass the cooler in the radiator with an after market cooler.If the cooler is plugged or restricted you will see it in the overdrive housing (bolts on to case)or planets where the intermediate shaft goes through the case.