Okay, my little '98 Sport (Auto, 2-wheel drive) just started doing this.  Right after I start her if my foot is on the brake she kind of shimmies as if there is some kind of a vacuum problem.  It also happens when I am going in reverse and depress the brake.  The car doesn't really move in any forward or backward direction just kind of clutches like it's going to cough.  She has had regular oil changes and has behaved like a dream for these 120k miles we have shared together.  Anybody have a clue what might be going on??  

If I read your post right, this happens in gear, not rolling, when the engine is cold after just starting it up, right? If that's the case then I would suspect that it's got a slight miss in it, probably due to one of several things. It might have a loose plug wire or a spark plug might have come loose causing poor compression. It might need new plugs, wires, distributor cap, rotor or a couple of these. When was the last time any of these things have been changed? It sounds like you just need a good tune-up.

The weird part is that this only occurs when I step on the brake.  It doesn't just necessarily happen when it's just been started.  But yes, it only happens at slow speeds going in reverse or rolling slightly forward and occasionaly right after it is started, but only after the brake is depressed.  Her last tune up was only about 8 months ago and I just had the oil changed and trans fluid filled last week which is oddly enough when this started.

A leaking brake booster might
cause this too

I agree with Wild  . On mine when you press the brake several times like pumping it bogs down for a couple of seconds . Both problems (yours and mine) are probablly caused for vacuum leak or brake booster leak. There is  a small valve inserted in the vacuum hose that connects the intake manifold with the brake booster , it exactly is in the first piece of hose connected to the booster. Check if it is working correctly it should only let air go in one direction (away from the brake booster)

Now some people will tell you it's the Knutson Valve, or that the Muffler Bearings should be cleaned with turbo blinker fluid, but we all know those things couldn't cause your symptoms.

My guess, and it's just a guess, because I don't have a 98 - just a bunch of 95's, is that the OBDII port on the ECM is full, and you need to drain it.  Usually, a metric phillips gorge spanner is used, but I have had great luck, just whacking it with a big rubber mallet.  Now mind you, you have to be careful not to damage the flux capacitor, but shoot, that's a good two barleycorns away from the OBDII port!

Someone go tell the bartender that Bob needs cut off...


;DLOL

A leaking brake booster might
cause this too

We had this same problem, I had bashed the side of our brake booster with a BFH from inside the wheel well trying to make room from shock towers.  Damaged the booster, didn't realize it for months.

Replaced booster, problem went away.

(and it brakes much quicker too)
~Nate

Did you check to see if your front rotors are warped? It's happened many times to me, especially if you've gone into a cold stream or a snowbank when your brakes are warm. usually takes some time to devellop though. It's just an idea.

Now some people will tell you it's the Knutson Valve, or that the Muffler Bearings should be cleaned with turbo blinker fluid, but we all know those things couldn't cause your symptoms.

My guess, and it's just a guess, because I don't have a 98 - just a bunch of 95's, is that the OBDII port on the ECM is full, and you need to drain it.  Usually, a metric phillips gorge spanner is used, but I have had great luck, just whacking it with a big rubber mallet.  Now mind you, you have to be careful not to damage the flux capacitor, but shoot, that's a good two barleycorns away from the OBDII port!

Speedracer and I were just talking about this a couple weeks ago. He was having problems with port 47 on his ECM. turnes out that the intake filter was clogged. As a result no information could get in and was building up at the intake. It then leaked into port 46 and caused major havoc. He replaced the intake filter with a washable one now so the cross leakage is not an issue anymore.