PO300 Cylinder misfire

99 tahoe vibrates and loses power going up long sloping hills. Wires were replaced, spark plugs replaced, distrib cap replaced, fuel filter replaced. The fuel pump and sending unit are new. Check engine light comes on and code po300 random misfires reads cyl. 1 247times cyl. 2 1358 cyl. 5 731 cyl. 6 30700 cyl. 8 372. (not a misprint.) Have been trying GM injector cleaner. It helps slightly. But lately has been starting then dies. After that happens it is very hard to start. it has 97,000 miles on it. I have been going with the assumption it was the injectors, but after 3 bottles of GM injector cleaner at $18 a bottle i am not so sure.

It could be the dreaded fuel pressure regulator- if you can get a gauge on your fuel inlet line, see if the pressure holds after the ignition is off.
If the pressure drops by half in less than 30 seconds or so, that points to the FPR.