have you checked your throttle switch? It's found at the engine end of the throttle linkage. I've posted these steps many times to test the switch is working, but only to people with the 4-cylinder engine.
Your 6-cylinder is very similar (same bore/stroke but 2 extra cylinders). Give it a try
Drive at 30mph in '3', with foot off the throttle. Change autobox down to '2'. If it jolts down into 2nd instantly, your switch is OK in the no-throttle positition.
Now do the same thing, with just a very small amount of throttle applied (enough to unclick the switch). If the gearbox slips and generally dosent instantly jolt down, the switch is working fine.
Quite why the throttle switch connects to the gearbox as well as the ECU I haven't found out yet, but when off the throttle and coasting its designed to cut off fuel supply to the engine (better economy). If it's not working your car may chug. When cold, the fact that it's not working may not matter so much as the engine richens itself when cold, supplying extra fuel.
Let us know what you get..!
2.5-16v 1990 90,000m Astral Silver
2.0E 8v 1986 107,000m Black