1999 Chevy Tahoe

I HAVE A 1999 CHEVY TAHOE WITH 174275 MILES ON IT. I NOTICED A SMEELL COMING FROM MY EXHAUST, SO I TOOK IT IN TO BE CHECKED OUT. AND I FOUND OUT THAT I WAS OVERLOADING FUEL AS WELL RUNNING RICH. THEY ALSO TOLD ME THAT THERE WAS A DROP IN FUEL PRESSURE ONCE THE TRUCK WAS SHUT OFF. THE MECHANICS TOLD ME THAT I NEEDED TO CHANGE THE SPIDER, SO I DID. ONCE THAT WAS CHANGE THE SMELL WAS GONE AND THE OVERLOADING WAS NO MORE. SO THE SAME DAY I WAS DRIVING ALONG MY WAY, I NOTICED THAT THE TRUCK WAS MISSING PRETTY BAD. I WAS ADVISED TO RUN ABOUT 2GALLONS ON 106 OCTANE RACING FUEL AND A BOTTLE OF STP FUEL INJECTION CLEANER, SO I DID. HE ALSO TOLD ME BRING IT BACK TO HIM SO HE CAN CHECK AGAIN. HE POINTED OUT TO ME THAT MY #5 CYL WAS MISFIRING AND THAT WAS CAUSING THE MISS. I CANT UNDERSTAND WHY IT WOULD RUN SO BAD AFTER REPLACING SPIDER WHICH I THOUGHT WOULD REPAIR MY PROBLEMS. CAN U PLEASE TELL ME WHAT CAN I DO TO CORRECT THIS PROBLEM.