Chevrolet Repair: 4.3l 89 chevy pu, timing advance, 5th gear

Question
my 89 pick up has been dropping off in power in 2nd thru 5th gear under load or on no load. Sometimes it even backfires and lately I have had spark knock. I have replaced plugs,cap, rotor, and just the other day replaced the fuel pump. Still does the same thing. When I pull the codes it came up as spark sensor, changed it, no help. The timing is set correctly. Have also replaced the distributor.
It is not a constant problem. Does cold or hot.

Answer
I have to suggest timing.  I know you say it is correct.  What Man. Vac are you pulling?  13 to 15 is good.  Lower is bad.  Lower and consistant needle is timing.  Did you check the timing in basic timing mode?  That is when you unplug the timing advance wire before you check the timing.