1997 Chevy Tahoe P0303 code - Need Help!

Hello! I am new to this forum and appreciate all of the help I can get. I am trying to get my wife's 97 Tahoe smogged. The Tahoe has 150,000 miles on it with a 5.7 Vortec V8. It actually passes the tailpipe sniffer, but the Service Engine Soon light keeps turning on and won't pass because of it.
Before the Tahoe was taken to the smog shop, I replaced plugs, plug wires, distributor cap and the rotor. I disconnected the battery cable to reset the computer. It looked like everything was OK until my wife drove it about 15 miles, and the service engine light came back on. So she took it to the smog shop to have it checked out. He ran it on the dyno and tested the emissions. They were on the higher side, but would pass. He then pulled the codes. It had a code indicating that the second O2 sensor on bank 2 was bad. It also had a P0303 code indicating a cylinder 3 misfire. I changed out the O2 sensor replaced the #3 plug wire again. Disconnected the battery to reset the computer, and again the service light was off. She took it back to the shop where the light came back on. The O2 sensor was clear, but the P0303 code was back on. He also did a compression check and found that #3 cylinder had 125 PSI. I believe that is enought, maybe not great, but should be enough. I replaced the #3 sparkplug tonight and drove about 15 miles before the service engine light came back on. Any ideas?
Thanks,
Ron

Sorry for the "double post"... Another thing I need to add is I also ran a can of Sea Foam injector cleaner through the last tank of gas, but didn't help.

Ron

Next would be to do a fuel injector leak down test to make sure that each injector is working and spraying the correct amount of fuel, but this isn't easy for the DIYer to do, as you have to have a scanner with that capability, and the injectors are under the intake plenum.

Well, I changed out the #3 injector, as well as the upper intake plenum gaskets. I drove the Tahoe for 30 miles last night and thought that it was repaired. My wife was taking it to the smog shop this morning, and the check engine light came back on. I need to get a new code reading at this point, but assuming that the same P0303 code is there, what next? This is driving me crazy

Ron