Hmm. Given that extra bit of info, another strong possibility is a stuck EGR valve / bad vacuum modulator. The car is at the right age and usage to start seeing issues with the EGR system. EGR systems are mechanical in nature, so it fits the warm restart behavior. I've had cars that the EGR valve completely jammed with carbon deposits - some case jammed completely open, so it was redirecting a good portion of the exhaust gas into the combustion chamber - made the engine either immediately stall or slowly choke down, depending on the RPMs and load.
First - check ignition system:
- Definitely wouldn't hurt to pull the sparkplugs and "read" the ends. That should give you some idea of combustion issues. If the plugs look OK, double check the plug wires and cap/rotor. Want to make sure there is no carbon tracking or other arcing issues inside the cap/rotor. Same with the plug wires, visually inspect them for oil soaking, jacket damage, make sure they are making good contact with the cap and plugs. If you have a multimeter handy, check the resistance of the coil pack, hot and cold if possible.
- If those all check out, try to visually check for arcing with the car in a dark area (inside the garage is OK for a short time, make sure you crack the garage door to vent the exhaust - better to do it outside, if it is dark enough out there) - with the hood open and engine running. The arcs can be very hard to see, but after your eyes get use to it, may be able to see electrical arcs from wires to ground or even between wires. Long shot, but an easy check that anyone can do - this will catch those intermittent cases that are hard to diagnose.
Check the fuel system:
- Replace inline fuel filter if you haven't already done so - will be by the driver's side fender, metal cylinder with fuel hardlines running to it. Best to hit it with some penetrating fluid a couple days in advance to help loosen that corrosion on the fittings, flared wrench can help bite into the fittings better than a standard wrench - prevent rounding off the corners of those fittings.
- Doesn't hurt to check on fuel pressures at different RPMS. Should be a schrader type valve running to a fuel hardline - verify that pressures do not drop when you increase RPMs.
- Listen to the injectors "clicking" when RPMs are varied, should be a pretty steady clicking, increasing in speed with RPMs. You can use a mechanic's stethoscope or a long screwdriver with your ear to the handle. If the sound is not even or missing "beats" - could be a sign of a failing injector.
Check vacuum sources:
- This will include the EGR system, brake booster, intake manifold lines, fuel regulator, PCV, EVAP system, etc., any source of vacuum. A vacuum gauge might also be helpful here - can pickup on vacuum leaks that cannot be visually found. Fortunately, the majority of problems in this area are pretty quickly seen visually - look for signs of damaged/missing vacuum hoses. Should be a diagram under the hood somewhere. Old school trick is to hit vacuum unions, around gaskets, vacuum tee's with a spray solution (I use plain water with a couple drops of dishwashing detergent) - spray suspect hoses with this and if the idle jumps or greatly improves - you found your leak.
