'96 Tahoe - P0305 code

I took my '96 Tahoe (V8, 5.7L) with 170,000 to Auto-zone to have the engine codes checked after almost 2 years. I got P0135, P0141 and P0305. I checked out and wound up changing out the O2 sensors indicated by the codes and the codes were cleared. I changed the plugs (AC-Delco) and wires (OE-type) but the P0305 code stays. I was checking around for leaks and I noticed a whistling sound from the right side of the fuel injection (facing the truck). My next thought was to change out the distributor cap but I didn't want to keep throwing parts at it without getting some feed-back from you guys. Your input is always appreciated. Help...! I need to pass emmisons or face having my tags suspended. Thanks again.

Check for a vacuum leak on cylinder 5.
Any vacuum lines or hoses off /unplugged?
Find that whistling sound and you may fix the problem.

Thanks, MT. I changed the distributor cap and rotor and the whistling went away. I didn't notice any vacuum leaks. I took my truck back to Autozone to clear the code (again), and on my way home, the SES cam back on. I'm pretty sure it's the dreaded P0305 codes again. I think I'm going to take it to my mech. tomorrow to take a look at it. Thanks again for ALL your help. Thanks to you, I was able to change my O2 sensors, clear those codes and learn some stuff in the process. Thanks again MT...!!!

You are welcome and thanks for posting back with how it went.
And let us all know how it goes at the mechanic.
Good Luck MT

I took the truck to my mech. (gulp) and he replaced my intake gasket. It was pretty cracked up. I guess after 11 years it was time. I drove around for about an hour once he was done replacing it and NO SES light as of yet. Also, I can feel a difference in how she starts and rides. She's rides and idles extra smooth now - like she did when she was new. Thanks guys (MT) again for all your help...!!!