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I have a 1997 Chevrolet C1500, 5.7L with 200,000 miles. The truck has a loss of power past 1/3 throttle and the engine misses a couple of seconds after the transmission down shifts while still accelerating. I have replaced the following: Fuel filter, air filter, distributer cap and rotor, plugs and wires, TPS, and Mass Air Flow sensor. I have taken it to a mechanic and they told me the fuel pump has good pressure. They thought it was the Cat. Converter, so I took it to a muffler shop and after pressure testing the converters, they said they were good.
The truck runs fine but the loss of power makes it difficult to pull a boat or accelerate to pass a car. The problems started when the wire for the knock sensor was disconnected for a couple of months and once I finally tracked it down, the knock sensor code cleared out and did not come back. I can get the engine to pull two pre-converter O2 sensor codes when I push the pedal all the way down and hold it there for a while. When I do this, the engine runs at around 4000 RPMs and sounds good but will not accelerate. I have also gotten a post-converter O2 sensor code and when I replaced the sensor, the same code came back a couple of days later. It reoccurs every 3-4 of months but I just clear it out and it goes away for a while. Could the knock sensor being disconnected for a while have damaged the engine? Anybody have any suggestions?  

first.  were the intake manifold gaskets changed???? as they are notorious for the original ones leaking coolant and vacuum.. coolant mostly...

if so... who reinstalled the distributer..    it has the cam sensor in it.. the spark timing is set by the crank sensor under the timing cover...

with a scan tool... you can see if the cam and crank sensors are in sync..   if they are not.. it sure will not have any decent power..  if the distributer has been turned slightly.. it will take away all kinds of power...  as the cam sensor sets the injector spray timing...

i would also like you to examine the outside of the ignition coil.. i have had to replace a LOT of them do to external high voltage leaks..  you can see them at night in a really dark location. or take the coil and bracket off.. examine the outside of the coil. look for circular dust marks around the outside of the windings near the square frame..

if you have to remove the module.. be sure to put a fresh dab of dielectric tune up grease under it..  or it will fail..

if you get a chance... have someone with a OBD2 scan tool that displays live data  hook it up and look at the TPS (throttle position sensor ) voltage as you really slowly move the pedal...

some scan tools can graph the voltage signal... so you can look for spikes in it..  the TPS has been a problem on some of these also..  and at 200K +  its probably worn out..   it just has a layer of carbon film with a pair or gold plated fingers rubbing across it..  so they do wear out..

grounds...     the ecm is only grounded to the engine..   usually to a stud behind the thermostat housing..  if that is loose.. dirty.. corroded.. missing its ring terminal and just bare wiring wrapped around the stud..  it has to be fixed..

you might also want to clean the various ground connections at the engine side of the negative battery cable..  taking the fastener out..  using a stainless steel wire tooth brush to clean the fasteners and ring terminals..

these are things that i have to check on every vortec motor that comes through the bay doors..


as for the post cat code.. probably a P0420...  i would look at the operation of the air injection system.. and its check valves.. and for vacuum leaks on the intake manifold.....