1994 New Yorker Rough

I have a 94 New Yorker 3.5L and it idles rough and when I accelerate the car starts jumping hard and there is a popping sound. I popped the hood and you can see the engine shaking. I have checked the spark plugs and did the diagnostic test and it came up 55 (clear?), have added stp fuel injector cleaner and something called sea foam. Since it came 55 my o2 sensor is fine right? People have told me it could be my intake gasket or the catalytic converter because the exhaust stutters. How can I check these? Thanks in advance.

the 3.5 V6 has a history of bad intake manifold gaskets going bad usually around the number 5 cylinder runner. I'd start there first. Next make sure your plugs are getting fouled, from what you're describing sounds like fouled plugs or shorted/crossed wires.