4.3l engine sounds like 4cil..

hello..

i have this problem..
blazer s10 1990 rebuilt engine.. 4.3

ok guys , when i drive the truck and i press the gas peddal to the top, the truck run verrryyy powerful.. but after that my engine sounds like a 4cil engine powerless noise and the truck feels more soft (less weight)..
and runs fine but not too fine like before ..
so what can cause this??? bad gas, plugs ,spark plugs or something??
note: the spark plugs have a little oxide in the electrode.. like water in the gas...
please help me.. i hate this sound...

when i turn off the truck and let it off for 15minutes.. then start again start with the normal sound..and power..

help..
Nalgman