I have a 1995 SL2 with 102,000 miles that is not regularly used. After
it sits for a few days, it won't idle correctly. On cold start, the
idle settles at 700-750, but as soon as I use the throttle and then
back off, it will chuff along at 400-500 or, more usually, die. The
problem persists after the engine gets to operating temperature - when
I release the throttle, the RPMs plummet to 400 or death.

However, if I nurse the car through a half-hour or so of this, turn it
off and let it cool completely, then restart within 12-24 hours, it
idles pretty normally - cold at around 1300, warm at 850, and on
throttle release gradually declining to normal idle. It continues this
during periods of regular use, but if I let it sit for a few days the
problem returns.

I've recently replaced the thermostat, valve cover gasket, plugs and
wires. There are no lights or codes in evidence.

What should I try next?