As others mention, your first suspect should be the IAC. There's a really simple test you can do anytime, any place that's almost guaranteed to tell you if the IAC is involved (I do this as part of routine maintenance to spot any problem developing). While idling, tap the IAC valve a few times with the end of a stick, even a broom handle. If the idle changes (returns to normal) then it's a sticky/dirty/worn IAC. Remove for an attempted cleaning with carb cleaner (or even paint solvent), but be careful not to let it soak lest cleaner get into the motor, shake thoroughly a number of times, rinse to see what comes out, repeat and dry. (BTW, Phil, the LH 2.4 IACs aren't spring loaded like the LH 2.2 ones -the giveaway is that these use a 3-pin connector, not two.) Although you don't want oil residue in it to act as a dirt magnet, if needed you may want to try getting a good spray of WD-40 down to the piston bushing, let stand vertically to soak in a bit, rotate the piston back and forth to work it in (I just flick the whole IAC in my hand to do this) and give it a quick final spray of carb cleaner to remove the excess. For a bad case, I'll repeat this step using 3-in-1 oil to lube the bushing. Also for a bad case, I'll finish by using OBD DTM 2 on port 2 of the diagnostic connector to fully exercise (tic-toc) the IAC from end to end of travel to work it in and confirm operation (see the FAQ), also often temporarily improving its function if you let it repeat a number of times.

Second on your suspect list should be the throttle body. Best to remove it to the bench for a thorough inspection and cleaning (which I do every oil change), including the two brass nippled tiny vacuum ports (if needed, ream them out carefully with an undersized drill bit). The triangular gasket usually separates without damage if you're careful. If not then they're cheap (I always have a spare on the shelf, ordered with my oil filters, also a spare rubber ring for the oil filler cap and a copper crush washer for the drain plug). In a pinch you can cut your own paper gasket from a cereal box. Another thing you can do while idling is spray carb cleaner (or starter fluid or water) around the end of the throttle shaft (under the spring) to see if the idle changes, indicating an air leak at a worn shaft. A little wear in the shaft is normal in these old cars, you just don't want a significant leak that can make for a slightly unstable idle, also this is unmetered air intake not seen by the AMM which makes for lean running conditions (the O2 sensor may not respond quickly enough to adequately deal with this).

Third is a plugged PCV system, something that's often neglected until it becomes serious. A dirty PCV system may include a plugged flame trap element ( NB turbos don't have an element in the flame trap), a plugged small vacuum line (connected to the flame trap) also hoping the oil separator box isn't plugged (difficult to check, if suspected then remove the oil filler cap and try using a hose to see if you can blow any air back into the oil box, if not then it may be completely blocked). Every other oil change I routinely check and clean the whole PCV system.

Fourth, check all air/vacuum hoses for splits at the ends and at the bends (especially the small vacuum line that attaches to the flame trap). Remove and carefully bend and check the accordion air intake tube for splits (often out of sight underneath).

After dealing with all those regular maintenance items, please report back and we can give you further guidance if needed
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now