Misfiring

I have a 2000 Chevy impala 3.4 it will misfire but not all the time.

2000 chev impala started missing at any speed or rpm. Drove it several times during the day and when I came out of the store it was missing badly. Any ideas

I went to my mechanic today and they only had time to do a basic code retreival, but basically my 01 impala LS, 3800 v-6 series II, 121,000 miles, all four cylinders are misfiring at different times, but no. 4 is really bad. It only runs funny when I am accelerating, they said it could be as simple as spark plugs, but as serious as intake manifold. He indicated he was leaning toward ignition, or a coil going bad. Any thoughts or extra insight for me before I take it in tomorrow? It ran fine before yesterday morning.

My 2002 impala starts to skip

Recently my Impala started losing power when RPMs get around 4000. The first time it happened the check engine light flashed on/off, which the user manual stated as misfiring. The engine would not rev past 4K and pushing the pedal down further had no effect. The light only flashed and did not stay on after RPM's dropped below 4K. A computer scan showed no codes.

It still is happening, only when above 4K RPM, but the light no longer flashes. Below 4K the car drives fine. Still no codes in the computer and the light is still off.

Recently I put a bottle of Regane injector cleaner in the a full tank, and after that tank was empty refilled with 93 octane (typically I use 87-89 octane gas). I'm not 100% sure the problem started after this but it's probable because I never noticed it before the cleaner.

If it helps anyone, the problem ended up being the catalytic converter. According to the mechanic, there was a stored misfire code in the computer, and when the engine misfired it was probably running on too rich a fuel mixture (possibly caused by the injector cleaner?), Which fouled the catalyst.

Luckily, this was covered under the EPA emissions warranty (8 years/80k miles), so I didn't have to pay for it. :)