[Mark Yacavone]

There's only a few things that can go wrong with a converter. Most of them are very noticable.
If the sprag doesn't hold or slips off the line ,stall speed will decrease dramatically.
If the sprag slips downtrack ,it won't do it consistently, so the numbers in the middle will vary.
If a pilot thrust bearing goes away, it will increase the clearance between the pump and the turbine and the stall and trap RPM will most likely increase.
Same for broken fins , but you'd have to have more than one or two broken.
In the last three cases mentioned , you'd have metal everywhere in the trans and the whole deal wouldn't last long.
My guess: The converter is not your problem.