catalytic converter

i just had cat convertor installed for emmisions . went into garage running perfect. after installation i tried to leave lot it stalled twice. when i got on road it started run very rough ,would not idle . took on highway at around 2000 rpm it started surge then rpm would drop. black smoke coming out tail pipe

Kinda sounds like a faulty cat, if you can get it to idle put your hand behind the tailpipe and feel for the exhaust coming out it should be a steady firm pressure/pulsing, if you cant feel this or the exhaust is barely coming out thats a good sign of a clogged cat.

Also after driving or idle for a bit the exhaust manifold, cat and downpipe should be alot hotter than normal, many times those parts mated with clogged cats can get glowing red hot.