Trouble starting

[Benjamin Weste Pearre]

Problem of the day:

The Gimp (1990 TQA) has trouble starting.  Sometimes.

The problem: the engine just doesn't seem to catch when the starter
motor is turning it over.  Or, it will catch after about a second, but
start turning over too slowly for the tach to measure, stumbling or
hiccuping or whatever.  Sometimes it sits in that state for a while,
and sometimes the engine just stops.  If I give it a little gas, it
usually (not always) jumps up to higher RPM, and is completely happy
thereafter (or perhaps I have to give it a couple of shots of gas).
I've had to sit on the ignition for up to 30 seconds to get going.

When: only sometimes, with no discernible pattern yet (if it's
affected at all by air temperature, the effect is minimal).  Usually a
problem in one of two cases: 1) the car is cold, or 2) the car was
fully warmed up, shut off, and has been sitting for more than a minute
or so (immediate restarts seem to work fine, and I've had trouble
starting after a minute or two, waited 15 minutes, and had no trouble,
but the problem is not consistent enough for that to mean much).

No fault codes.

The problem is very recent (last week or so).  Recent work: replace
throttle switch and MM hose, and do lots of WOT runs to make sure that
I now have full boost ;)  The timing is roughly right for these to be
correlated, but I don't see a mechanism.

I have _most_ of the maintenance records, and it _appears_ that the
oxygen sensor has about 120000 miles on it.  I'd hazard a wild guess
that it'd be worth replacing and might even solve my problem?  Do
y'all have a better idea?

Thanks enormously!
-Ben

--
Ben Pearre          1990 200TQA          http://hebb.mit.edu/~ben