Problems with power

Someone please help. I have a 1996 Chevy S10 Blazer 4.3 4wd 4 door blazer. Lately I have been having problems with the power. I can't seem to get it
up to speed on the Highways and up hills without it down shifting into 2nd gear bringing the RPM's up to about 3500-4000. I replaced the spark plug wires, rotar, distributer cap, and fuel filter and this does not seem to have helped. Does anyone know what could be the problem? Also, there were a couple of different auto parts dealers that stated a specific type of spark plug has to be used on this vehicle and some say it doesn't matter. Does it or does it not and if so, could this be the problem?
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