I'm no expert by any means, but I thought my '85 SS Monte Carlo was slipping in and out of 4th when it was really the torque converter locking or unlocking. It has a 200 4R but I think they both use the same type of lockup converter. Another thing that may make it slip in and out of 4th is the throttle position sensor cable could be out of adjustment if it slips in and out of gear when you try to accelerate.
One way I found out my torque converter was locking and unlocking was by disconnecting the electrical wiring harness from the transmission that controlled the lockup solenoid. I ran the thing unlocked for years before having the whole thing rebuilt when it needed other repairs.
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