Cats fail suddenly all the time, heck I've had 2 do it, Jeeps had horrible problems with this in the mid 90's driving down the road and pop thunk no power..

Not all additives are snake oil, Sea foam is like almost a hundred years old and boat owners use it religiously to prevent water separation in the fuel. The key is don't think more is better. Use it as the directions tell you.. It will raise your exhaust temps about 150 degrees because it has some pretty good solvents in it.
Techron is another one that is effective at getting water out of the tank, and its recommended by Porsche, Ferrari, Bmw etc.. If you got a really big load of wet gas an additive won't help, IE like a gallon of water in the tank.

Assuming you've eliminated engine sensors,,, and you really think its the fuel...
Most engines have a fuel pressure test port on the fuel rail, and most fuel pressure gauges for EFI have a pressure release valve, one way to empty a tank is to attache the gauge, open the vent with the hose in a fuel can , pull the fuel pump relay and connect a wire from the battery to the fuel pump connection on the relay, use the pump to empty the tank. Worst case you need two 5 gal cans.
Be aware that if you got a load of fuel that is that bad, the in tank strainer can get fouled as well, so once a fresh can of fuel is in there if its acting like it can't run above idle it could need a new strainer and or fuel filter.

 

My mechanic checked it out, ran all kinds of tests/diagnostics and found nothing. Spark plugs, O2 sensors, c-packs are all new. The catalytic converter is not clogged (But that does not mean that it may not be bad. It WAS in Florida until about 4 years ago, so I wonder how much the salty sea air erroded things). However, the car still does it between 3-4k/rpm. I can get up to speed as long as I keep it below 3k/rpm. My check engine light, just at this very moment, came on, but is no longer flashing. It also just jerked a little when it shifted.
He is thinking that it may need a lash adjustment. Other than that, he's at a loss (I've never seen him at a loss). Any other suggestions?
-Laura

 

If it has more than 80K since the last valve adjustment it needs done anyway..

 

Were coil packs the ones recommended? (OEM or Hitachi part # ending in 053)

Also if you say parts are new, when were they put in? (Time and miles)


Many times (especially with the cheaper aftermarket coils) they start to malfunction a few thou' down the road.

PS, X2 on the valve lash...