Where to start?

[mbeach]

This comes from Bob in response to a similar Q:

Sounds like a Vacumn leak to me. Make a good visual - CK PVC at valve & rear of Plenum also EGR connection at back of Plenum also Vacumn line at throttle body and at fuel regulator (Fuel rails Driver side rear).If all else fails spray Carb cleaner with engine running at intake manifold gasket mating services if RPM rises Try tightening manifold, if that fails new Intake Gaskets req. order
PN 8-97237-538-0 two req at $18 + each - 1-800- 727-8066 ask for Merlin. S&H is free

Torque value is 13 FT LBS on all intake manifold nuts & Cap Screws.

I had a similar surging upon hot starts and found it to be a vacuum leak at the intake manifold gasket. Just to make sure, I stuck a length of 3/8 hose im my ear and used it as a stethoscope to find the exact point of (air) entry.
Depending on your VX's in service date, this should be a warranty fix.

As for the cat, a 'rattling heatshield sound' can come from INSIDE the catalytic convertor body (as Tone mentioned), this usually happens after the matrix has been blown out and the internal shielding no longer has any material to keep it in place.
Get your in/out pipe diameter and overall length then order an aftermarket (high flow) replacement cat for <$125. I'm a big fan of Random Technologies' units. Have it welded in for $20. Catco units also come highly recommended, but I have no experience with them.

It's easy to get frustrated when so many things are going wrong with your truck, but when you look at the root cause of these failures, they all point to one, easily repaired problem.
Bad intake manifold gasket(s) = lean fuel/air mix (weird idle and CELs 017x),
Lean fuel/air mix = hot combustion temps,
Hot combustion = hot exhaust gas temps,
Hot EGTs = burnt O2 sensor(s) (and CELs again),
Burnt O2 sensors = burnt catalytic convertor,
crumbling cat = rattling noises.

Best of luck and keep your chin up, this board's value lies in its collective experiences with these same troubles.

Edit: I just noticed that you had an Injen intake. I don't know about the VX for sure, but there have been troubles with aftermarket CAIs on quite a few other vehicles. They cause the engine to run lean, even beyond what the ECM and fuel injectors are capable of managing. Just a thought.