2001 Chevrolet Silverado • 187,500 miles  Recently (before and
after my transmission rebuild), I've noticed a problem with my
truck not having the power it should at higher RPM's. The truck
will start, idle, and slowly accelerate fine, but once the tach
goes above 3000RPM, there will be a light rattle from the engine
compartment which increases depending on how hard you're
accelerating or how high the RPM's are. Following that 3,000RPM
mark, the SES light will flash until the truck either shifts or
you let off the accelerator. After about 30-60 seconds, the
light stops flashing, but may stay on due to a couple other
codes. This never happens when parked/in neutral, it only
happens under load.  The truck has brand new AC Delco iridium
spark plugs/wires, brand new fuel filter, clean air filter, and
of course the transmission rebuild with new torque converter.
For a while, I've been getting P0300 (random multiple misfire)
and P1133 (heated o2 sensor bank 1 sensor one, which IIRC is the
rear o2 sensor), but occas