"Chugging" during acceleration

Hey gang! I know I'll be able to find someone in a group this size and well- informed to help me.
I've got a 1999 Chevy Z71 Ext. Cab with the 5.3 V8 and about 176xxx
miles.
Several months ago after driving a long way on a hot day I noticed it would "chug" when accelerating between 40-50 mph but only going uphill. I did not have a load other than myself and a couple of tools tucked up under my backseat. Even if the grade was about 3-4 degrees it would still "chug." (by "chug" I mean it hesitates and lurches, hesitates and lurches at fairly rapid intervals) If it is a flat grade or any other speed (even if it is more than 50 mph) it does not "chug" at all when accelerating.
A couple days ago on a cool fall evening I was accelerating out of town on a black top and noticed it was doing it again this time a little more "staccato" (for those who understand musical terms...if not Google it) and the grade was about 5 degrees (just guestimating). Still no load in the truck. Keep in mind that this is not an on/off ramp type climb it is a matter of a few degrees uphill.
Is this a timing problem or perhaps a fuel line problem? Something serious? Something a quick maintainance procedure will fix? Expensive? Cheap? You get the idea!
Thanks in advance!
G
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I'd have the fuel pressure checked for starters, 60-65 psi, this assumes the plugs and wires are in good shape.

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My first suspect would be the fuel filter.....partially blocked it will give these symptoms under moderate load. Cheap and easy to do. Also rules out the easiest more obvious cause...fuel starvation under load.
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I had the same problem. turned out to be the fuel pressure regulator and needing a new filter.

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So it WAS something to do with the fuel line as I suspected. I just put on a new fuel pressure regulator maybe 6 months ago so I reckon that should be fine? Fuel filter was change with the fuel pump and that was about a year ago. Does it need changing again? I'm extremely novice with autos so is this something where I'd be better off to let the mechanic do it? Thanks for all of the advice so far!
G
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As I said get the fuel pressure checked for a proper diagnosis.

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Another possibility could be the catalytic converter is starting to plug up. Had the same symptoms on a 90 Pontiac - turned out to be the converter.

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