'03 Yukon XL 5.3L flex fuel low power rough idle P0300 P0101

I have a '03 Yukon XL 5.3L Vin#Z 103,000 miles. It has rough idle, low on power & misfiring under load, solid check engine light at idle and flashing while driving. P0300 and P0100 codes set. Engine temp. sensor reading 199 degrees. Replaced spark plugs, checked spark on all cylinders - good, replaced MAP sensor, swapped out MAF sensor with known good unit - no change, checked ignition 1 voltage 12.28v, checked 5v to MAP 5.02v, MAP low reference side is grounded, checked grounds to MAF and MAP - good. Found no restrictions or leaks from air cleaner to intake manifold. Fuel pressure 52 psi KOEO, Fuel pressure 42-48 psi flucuating rapidly with engine running, fuel pressure leakdown for 1 min. in spec. - 2psi, pulled 13 inches mercury vacuum on pressure regulator - in spec., no gas leaking from pressure regulator vacuum port. Engine Vacuum is solid 18 inches mercury. Should fuel pressure fluctuate rapidly 6-8 psi when engine is running? Any idea's on what to check next would be appreciated?

Update: Determined that one catalytic converter was clogged which was causing severe misfiring and lack of power. Replacing catalytic converter assembly solved the problem. Also replaced fuel pressure regulator to fix the fluctuating fuel pressure when engine is running and replaced fuel filter for good measure.