LS Experts....
 

07 Envoy denali 5.4 with AFM/DOD
220k
Was running great, started getting more intake type noise one morning soon followed by loss of power/driveability.

Had a shop look at it and they said computer took a crap. Swapped the computer and no change. codes for random miss and purge valve, also crank position (from new ecu)
New computer made no difference whatsover.

it will start, run a bit rough, rev once or twice and runs much worse then idles rough and has no change when accelerator is pushed. shut if off for a while and same thing happens.
no new codes, thought the decreased power engine light comes on. Sometimes it misses hard some times it just lays down.

Any ideas or just shoot the damn thing.......

I'd get back to basics, such as compression and leak-down test? Also check the plugs, wires, fuel pressure need checked?

Check the wire harness at each sensor for a good ground and correct voltage. Im guessing a bad ground or a mouse ate the wire harness.

Check fuel pressure, at 220k you may need anew fuel pump. My suburban didn't last that long. HP Scanner from HP Tuners is the best money I ever spent it will show you everything that's going on. I have diagnosed problems the dealer couldn't find. I bought the full version so I could tweak the programming. It's amazing what a few small changes will do for drive ability.

The ground is a good start...My 2010 Avalanche did something similar just adding a ground to the body from engine block cured it. My moms 2008 Tahoe started acting goofy where it would unlock/lock the power door locks while driving and the dash would completely shut off and turn back on but engine never missed a beat. Turns out the main power lead going to the fuse box has its own maxi fuse and it was corroded and making intermittent contact.
Next i would do a compression test....maybe (hopefully not) one of the intake valves stuck open if your getting intake noise....