'93 240: starts fine, then rough idle, then fine

[hackneyed1]

Here's one for the hive mind:
My car starts fine (although much quicker after sitting for a few days), idles fine for 15 seconds and then sputters like its losing a cylinder, then will smooth out within a minute. Maybe related, it also idles low (just under 500rpm all the time, slightly lower in hot temps) and will stall on startup when doing alot of shut off/restarts, say when running errands.
I have checked the IAC, checked the valves for tightness and recently replaced the in tank pump. The throttle butterfly was cleaned too.
thanks.

[volvolugnut]

Have you checked for vacuum leaks in the intake hose and vacuum tubes. These can all develop leaks over time. The intake manifold gasket can also leak.
volvolugnut