5kcstq engine problem when cold

[Wallace White]

Listers -

On two occasions, I've had a engine problem in my 5kcstq: the engine
idles extremely low or tries to die. It might idle at 400 rpm,
increasing a little as the engine warms up. It's not only an idle
problem, though. While driving, if I lift my foot, the deceleration in
gear is more sudden than usual and the stock boost gauge (I know,
inaccurate) read 0.0 bar. (It usually reads 0.2 bar or so near sea level
when decelerating in gear.)

This happened on two chilly evenings, a few weeks apart, but hasn't
happened otherwise.

I would think it was a vacuum leak except for the deceleration behavior;
my previous experience with vacuum leaks has been that you only notice
them at idle. The intermittent nature of the problem makes it hard to
diagnose, too. Any suggestions?

- Wallace
  '87 5kcstq 167k