I was driving about 60 mph on freeway for about an hour and engine started to sporatically miss or stumble. I pulled over and catalytic converter was smoking. A mechanic said it was clogged, so I removed it, but no fix. The stumbling still occurs, at all speeds and sporatically. Some times its fine, sometimes it stumbles. I've also changed the plugs and the purge valve and the oxygen sensor. This is a 90 GT with a 3.1 V6. There is no engine light and a diagnostic showed no codes.

Im thinking you have got a fuel injector getting hot and shutting down. Injectors going bad are very common on GM 3.1L engines.
When it startes shaking, pull over and do a cylinder balance test. That is pulling off spark plug wires one at a time and looking for no change in idle speed. If disabling a cylinder does not affect the idle, that is your problem cylinder.
Go from there - checking plugs, wires, and injectors.




You removed your convertor? Naughty, naughty! :nono: :naughty: