Could start by checking the plugs and COPS on cyl # 3 and 8 .

 

Are you thinking 5 COP failed simultaneously, and yet do not consistently run a misfire?

 

I agree with Logan. This appears to be an ignition problem and with that much mileage on the current COP's and plugs, it's time for a tune up anyway. Running full throttle will identify problems that existed in a relatively high mileage engine, but didn't show up until pushed. Don't forget to use new boot kits, springs, and resistors (if used).

If tuning up the ignition system doesn't clear the codes, check your knock sensor. It's buried under the intake manifold, tough to get to, but will throw all kinds of ignition codes (false misfires or knocks) if it's not properly torqued, corroded or just plain worn out.

 

After a series of tests, live data scans, putting a scope on the coils, comparative cranking compression tests, cylinder #3 is the culprit. It has 100% leakdown and 0psi compression. It appears to be losing compression through the exhaust valve.

I have a set of PI heads, so I'll be dropping them off at the machine shop to be checked out and the heads will be coming off for further inspection.

 

Thanks for the update. I wonder why you burned a valve? I would have a close look at the ignition and injectors, especially #3, while apart.

 

Also have a good check for blown head gaskets as well as burnt / sticking / bent valves on strip down .