Running in 3rd gear the torque converter (TC) lockup occurs about 40 mph. When in OD it occurs at 60 mph or above. In OD the lockup sometimes gives a jerk plus it now seems to hunt at around 60 mph.
The shop ran the diagnostic tool while they took it out driving to witness the above operation. They tell me that the lockup is a gradual engagement, that the solenoid is pulsed (PWM) over about a 10mph range until it's fully engaged.
I just picked the car up and was handed this ATRA Tech Bulletin #1026 relating to the 4L60E and 4L80E transmissions dated August 2006 titled: No TCC apply, TCC apply not commanded. It reads as follows: "Also some GM trucks will not command TCC apply until vehicle speed exceeds 65 - 70 mph. This has been observed on otherwise normally functioning vehicles with no DTC's pending or present.". The shop says that the RM also uses the 4L60E transmission.
When I first bought it in there was a misfire code in the RM's computer. The code was removed and did not return while the transmission continued to engage lockup at > 60mph after being driven extensively.
I will say that while driving the car home at between 35 - 50 mph for about 15 miles, I could feel a roughness in the engine. I think I'll take the car down to a Buick dealer and have them look at the engine (with your suggestions), because at this point, the transmission was rebuilt but the initial problem with it remains. The rebuild included replacing the TC, solenoid valves, transmission ground wire (sometimes a cause of erratic operation), rebored TC solenoid valve body and new valve - anything and everything that could have been the problem.
Rob.