This IS a classic sign of a vacuum leak on the Dual Overhead Cam Motor (DOHC).
Especially the lower intake manifold gaskets as they meet the head, the PCV valve pipe on the drivers side rear of the rear cylinder head, or the brake booster vacuum hose.
Concentrate spraying carb cleaner on these areas when warmed up to see if the idle will change. Be sure to stay away from any ignition source, as the spray is VERY flammable.
Problem with my 1995 Monte Carlo Z34.
3.4 DOHC.
Work done to repair this problem = Replaced
Idle Air Control Valve.
My 3.4 idles VERY rough and dies cold. When warm the idle is somewhat erratic and although a little high it is tolerable. I cannot find any vacuum leak which is what I suspect. I replaced the idle air control valve and am now suspecting the EGR (it is digital). I'm not sure if it can be cleaned or not...... Any suggestions?
Problem with my 1997 Chevy Monte Carlo LS.
3.1L V6 engine
Check engine lights flashing and
OBDII codes p0300. Change all  sparks plugs and wires. Why does my check engine light have this code p0300. We   replaced coils and spark plug wire.

First you need to find out which cylinder(s) is misfiring.
If plugs and wires were replaced, another common cause for misfiring is a   burned /
corroded ignition coil. That should have been obvious while  replacing the plug wires.
3400 V6 Engine Misfire Repair Trick
Fuel injector problem causing misfire code P0300, P0301, P0302, P0303, P0304, P0305, P0306 on GM 3.1L and 3.4L V6 Engine. Also know as 3100 and 3400.
1999 Chevy Monte carlo  3400 V6 SES light problem.
Finally got a OBD 11 tester and i have a P0300 and P0302, or a misfire in #2 cylinder. Rechecked plug and wire, replaced wire, replaced coil for that cylinder. Sounds a bit better but still idles like a misfire, tester shows a misfire that is the only bad cylinder whats next, resistance on the injector, or would that have shown up on the tester?

Thank you for the tip / donation.
Assuming there is no engine mechanical problem with that cylinder- like a rocker arm stud has pulled out of the cylinder head, most probable cause is the injector. Scanners dont detect specific problem (injector bad) just the type of fault. Our injectors today dont usually fail because of low resistance anymore. They almost always fail because they are plugged.

Here is a little trick, with the engine at idle, and missing, take a socket extension and tap the injector. Very lightly at first a few times, then a LITTLE harder if it has not cleared up. Be careful not to tap too hard and crack / punch hole in the injector. This procedure will almost always free-up a stuck injector.
If it does, it may quit again in a day or two, or may last a month- hard to say.
If the injector does free-up, would suggest an injector cleaning service and fuel filter after replacing the bad injector.

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