99 taurus low rpms

sometimes when i start my car it kinda bogs down and the rpms are running at about 500 instead of the usual 1000. there is a strong carbon smell. and i've noticed that directly beneath the muffler on the ground is a black spot. when i give it gas it sometimes just goes back into it's regular rpms and runs smothly. if i let my foot off the gas it slowly sputters, shakes then dies like and old cartoon character.
i took it in to replace the cad converter but the mechanic said all he had to do was clean it out and i didn't have to spend as much. so he took all that hard grey stuff out (looks like be hives). i recently replaced the plugs and wires.......any suggestions?

That hard gray stuff is the catalyst in the catalytic converter. If the catalytic converter was breaking up before the repair, you might have a partially clogged muffler. How do the spark plugs look? Are you running rich?

replaced the plugs 3 months ago. i don't know if they have ever been replaced before then.
clogged muffler....how do i go about unclogging it? or do i just get a new one welded on there?

Take off the muffler and shake it out. If that doesn't work and if it is clogged then you will need a new one.

The Air Idle Control gets dirty on the 3.0 and it will not idle or may race when started.
Remove two bolts and use care not to damage gasket.
Clean by turning upside down and fill with carb/inj cleaner. Move plunger back and forth with small scredriver. Do this until no more black soot comes out.
Then do with W/D 40 to lube.
Also Pry off the vent cap on top and clean the round cotton like filter.

thanks! i'll try that this weekend. being low on funds, this should help alot