Transmission intermittent shutter and hard shifting issue?

Car in question is a 2000 Buick Century 3.1L V6 and a 4T65-E automatic transmission. No idea how many miles because the odometer doesn't light up.

A little back story... car had said problem before, however it was because the transmission failed internally. I'm told the used transmission that replaced it only had 51k miles on its clock from the previous vehicle it was salvaged from.

Well, the owner of the car tells me that about a month after the transmission was replaced, the car started exhibiting the same problems again and the check engine light came on. I pulled trouble code P0742 (Torque converter clutch solenoid circuit stuck on). Knowing the transmission was recently replaced, I focused my attention on the external circuitry between the PCM and the transmission connector harness. I performed the tests for the involved circuits by the book and found no faults or issues.

I should also note that the problem is intermittent, and during my attempt to diagnose the problem, I wasn't able to reproduce the symptoms. I am aware that these transmissions are known to have the line pressure control solenoids go bad, however my scan tool is not showing any codes for that. I was also told that the only thing that was done to the replacement transmission was adding the fluid. I'm wondering if its possible that the filter is clogged up or something, or it just maybe I should bank on having to change the possibly defective solenoids. What avenue should I try?