Mercedes: 1983 300 series, catalytic converters, catalytic converter

Question
My 1983 300 series runs fine at startup, but wont go over 30 after it warms up.  Tow trips to the mechanic and the problem still exists, iny ideas?

Answer
Where are you?
Sounds like you need a new mechanic...

My first reaction = plugged catalytic converter???
Disconnect the exhaust from manifold, strap/support the exhaust away from the manifold, road test around the block = if the issue is gone, the catalytic converters are bad.