I have a '97 chevrolet suburban 2500 series with a 5.7L (422,+++) and no current upgrades since we purchased it in '03.

I've noticed in the past couple of years that it seems to struggle at times. You can get it up to 70 mph and set the cruise control and sometimes it will run fine, others it will run for a bit and then begin losing speed. The transmission will downshift into passing gear, but the vehicle speed will continue to drop until the cruise control disengages itself. At that point, no matter how far you depress the accelerator, it won't gain any speed. Sometimes if you let off the accelerator it will slowly creep back up to the 70 mph, but it struggles to get there.

I have also noted that if you stop for a meal break, about an hour or so, when it is struggling like that, when you come back out and take off it will run fine for a long time before it begins the struggling again.

Check to see for any pending codes.

When was the last time you did a tune up?

Agreed, check for codes. Those symptoms could also be a plugged catalytic converter among other things.

Codes are the first step though, as most anything will trip a code of some sort.

What does the oil look like? My 1999 Suburban K1500 5.7 did something similar (albeit one time) when it ate its second set of intake manifold gaskets on a long drive, dumped coolant into the crankcase, and pressurized the crankcase. "Pressurized?" you ask? Yeah, when I got it pulled over, lifted the hood, pulled the dipstick, and chocolate milkshake squirted out of the dipstick tube for three minutes solid.

Not saying you have this severity of an issue, but you might have something similar starting up.

Also:

Odometer?

Mileage on spark plugs?

Are catalytic converters original?