[blabhern]

clogged cats should give you an SES light and a trouble code. With the symptoms you are describing,I would suspect a fuel pump that is going bad, clogged fuel filter, injectors, etc.

Try this out:

Get a can of fuel injector cleaner.
With the engine running pull the power brake vacuum hose off and allow the vacuum to suck in the cleaner.
Once all the cleaner is sucked in, shut the truck off.
Leave it off for approximately 15 minutes.
Turn the truck on.
It will smoke for a while until all the cleaner and whatever it dissolved burns away.

I've done this to my 96 k1500 5.7L and it smoothed out some rough idle and sluggish performance it was having at the time.

Hope this helps.