The engine should idle at about 500 - 700 rpm. The "problem" is that it
doesn't; there's nothing wrong with the transmission. Fix the idle and
the problem will be fixed too.

Different models have different ways of controlling the idle speed. Some
have an electronic module and their circuit boards dry out and the
solder joints crack - so one day its fine and the next its not.
There are some rebuilders of such idle speed controllers - if that's
what's installed in your model.

Fairly easy fix, but I forget how (I know how if yours was a 82 or
later but the 79-81 are a little different).

You need to replace your idle speed control computer... it is located at the
middle in between your two firewall when you open up the hood. Very common
problem.

Bare with me on this one, gang - When's the last time your transmission
fluid & filter were replaced, Derek?  Has it been longer than 30K miles?
If so, I'd have the transmission serviced first before I spent anymore
money on anything.

I seem to recall having a problem with my tranny slipping in & out of
gear at a stoplight (with similar differences in idle RPM depending upon
whether the tranny was still in gear or not).  Upon servicing the
transmission I found some cork gasket material had broken off & lodged
in the filter intake - resulting in substantially reduced fluid flow.
Replacing the filter and fluid restored normal transmission operation
with no more delayed shifts into gear from a standing start, and an idle
speed consistent with being stopped while in gear.  Reading through your
posts, I'm wondering if perhaps your problem is the same as the one I
just described?  Problem may be neither the transmission nor the idle
speed controller, but just a clogged tranny filter.  Same symptoms.

Jerry Wolfram
'78 450 SL - 214K miles