'96 LeSabre - all-round rough running

[Dr. Pimento]

I've got several interesting issues with a 1996 LeSabre:

Firstly:
When I start the engine, the idle is very rough, and nearly dies at several points. After about 15-20 seconds, it evens out for a moment - then the engine revs way up - as if I were accelerating at a rather fast speed (I'm not even in the car at the time, obviously). After just a few seconds of this, it drops back down to a "more normal" idle - still a bit rough, but nowhere near as bad as when it starts.

If I try to engage a gear (reverse or drive, either) during the initial "very rough" idle, the motor actually dies.

This symptom seems to be more pronounced after the car has been turned off for a few hours.

Secondly:
While accelerating at a moderate, steady pace, between the speeds of 20 and 40 MPH, the car will sometimes shift, rapidly, up and down, before settling back to "normal".

Thirdly:
While at speeds of 50 MPH +, sometimes (about once every three days or so) the car will shift, once, down and then immediantly back up (making the vehicle "lurch" pretty bad)

Fourth(ly?):
Frequently, when either setting cruise or manually maintaining a speed between 60 and 70 MPH, the car will develop a moderate shudder - to correct this, I can apply the brakes, lightly, to bring it down a few miles per hour, and then re-accelerate to my prior speed.

Fifth:
Very frequently, I get very sluggish acceleration response to pedal operation, at speeds over 40 MPH.


The "check engine" light triggered, and it threw the following codes:
1509 - "IAC System High RPM"
0113 - "Intake air temperature circuit high input"
0118 - "Engine coolant temperature circuit high input"
1298 - "Lean operation at wide-open throttle"


I've replaced the spark plugs and wires within the past 6 months, and recently replaced the water pump (it was leaking coolant at an impressive rate). The filter is in good shape. This vehicle has 173k on the engine, with no major work having needed to be done to it (aside from small part changes, like the pump) thus far.

I've gotten opinions that range from knock sensors, to speed sensors, to oxygen sensors.

Is there anything specific that I should be looking at?

[Dr. Pimento]

Updates - twice now the vehicle has cycled power - ie: the engine shuts off, and the dash lights turn on (as if I'd just turned the key to the 'on' position) - no brakes (assist), no steering (assist) - so I have to crank the wheel hard to get to the curb, restart the car, and go on my merry way.

I've replaced the wires and plugs - no help there.

At times, I can smell a 'sulpher' smell as I step out of the car.

A friend suggested it's the Catalytic Converter - any merit to that thought?

[maxwedge]

Get a scanner on it and look at these sensor readings, the iat sensor should read ambient air temp when cold, may even be disconnected, the coolant temp sensor should also read air temp when cold, the warm up to 190 or so. Start with these and post back what you find, you could also clean out the throttle body and remove the iac and clean that, make sure no vacuum hoses are disconnected also.