Just wondering if anyone can offer up suggestions for the cause of the following symptoms.

very rough idle in Drive when hot. (idles fine in Neutral and Park). Occasionally stalls at very slow speeds (up hills for example).

I have changed the Auto trans fluid for what is allegedly Magna compatible fluid.

Thanks for any ideas on where to look for a cause. I have the factory manual and the 800 pages just on the auto trans can be a bit tedious. Thanks.

Rough idle is not a gearbox issue. What is your logic there?

I think the issue is more likely to be low revs causing shudder of the engine, rather than a transmission issue. Our old Magna was fine in neutral and park, and used to shudder quite heavily in drive at idle, but once the car was actually moving or the RPMs were increased, the shudder went away. A lot of the posts i've seen around here seem to suggest wearing of the engine mounts as being the cause of the problem, but I never actually got the problem fixed so I can't personally vouch for this. You may wish to check for collapsed or sagging engine mounts. I'm sure some of the more experienced members here will also have their own theories.

Perhaps the ISC? do some searching for ISC or idle speed controller, I still have to go look at mine when I get some time...

ISC
TPS
Spark plugs.

etc
etc

Lots of possibilities. The gearbox isn't one though.

What is the idle speed in Drive? Does the idle go up or down when you turn on the lights or the air con?

My TR had the same problem as your car it turn out to be a dirty throttle body, haven't had issues yet :)

Thanks for the replies, on looking it appears that the idle speed in DRIVE can get down as low as 600 rpm. perhaps why it is coughing and spluttering ...

600 rpm is too low. Does the idle go up or down when you turn on the lights or the air con? If not, the ISC is most likely not working.When it failed on my 2.6 I used the throttle body screw to turn it up to 1,000 rpm hot. Totally different to drive, but best thing is to get a new one.