i just purchased a 1990 blazer s10 that had been sitting for 3 years. it runs a little rough. I put fuel injector cleaner in the gas tank and have changed the spark plugs. so after I let it run for 3-4 min the rpms start to drop low then come back to normal in a regular cycle. and it took pumping of the gas just to get it started. im not a experienced mechanic. any sugjestions on where to start

 

Try replacing the fuel filter and run one tank of high octane gas.