91 GMC rough idle

I have a 91 GMC Sierrra truck with a 305 engine with 260,000 km's.
It has been acting up for some time. In the last 8 months it will idle so close to stalling ,all the lights on the dash will come on then go out.It never does stall though. I can't say it always idles the same. Usually when I hammer the throttle a little,it acts worse. There are no trouble codes.It doesn't idle high ,so I suspect no vacuum leaks. EGR replaced last year and working. Distributor shaft replaced two years ago.(helped with spark knock) The coil is about 12 years old. It's kept pretty tuned up ,and I use injector cleaner often.
Can someone tell me what it can't be and what it can?

Here's what I think it could be..
Stretched timing chain
Worn cam and lifters
Worn out fuel injectors
Compression (different cylinder to cylinder)

I don't believe it's electrical because I have no codes, but not convinced it isn't by the different way it acts.
Any info is welcome.

Thanks

Kevin

I would try cleaning the IAC and throttle body first. That would be your cheapest bet. I had a problem with my 2000 Sierra, at idle it would almost stall then come back up and then down again. I changed the pcv valve and fuel filter, and nothing. Then I went and bought some Valvoline throttle body cleaner, and cleaned the throttle body and the iac and it quit doing it. Hope this helps.

Hey Kelsey

Thanks for the info... I replaced my IAC about 4 months ago because it was idleing up and down , even at operating temp. I also cleaned the throttle body first before replacing the part. The new IAC did fix the problem then , but I still have a very unstable idle now.
It seems throughout the regular driving range it's OK , then at idle it can act many different ways???? Could a leaking head gasket be causing this.I don't notice any oil or coolant leaks.
What else can I try?

Kevin

Do you use fuel injector cleaner? If you do or don't I would recommend a fuel additive called Seafoam. This stuff really works. That would be the cheapest thing to try first. Many things can cause a erratic idle. Sticky pcv valve, plugs, wires, fuel filter, air cleaner, throttle body, iac. Try cleaning the mass air flow sensor, with mass air flow cleaner.

"Seafoam" ...I hear this all the time on these forums and Motorbike forums , I can't seem to find any in Canada yet. I use lots of "lucas" fuel injector cleaner which I think is great stuff. I use it in every motor I own.
I'll try another cleaning with every component I can see... I also want to try checking my "cam lobe lift" to see if my camshaft is wore. Maybe my timing chain is toast too. How else can you check the timing chain other than taking the cover off and looking at it ?

Thanks
Kevin

I did finally find some seafoam here in Canada. It's at Napa Auto parts. I ran a whole can through with little change. I also used the stuff in my oil...again ,no change.
I was told to check my fuel pressure regulator. Does anyone know where that would be. A friend said check between the 2 fuel injectors. I haven't looked yet but would like to be looking in the right area.
Any other thoughts on why my truck is idling irractic?

Kevin