GMC Repair: Rough Idle, chevy tahoe, exact diagnosis

Question
I have a 1997 Chevy Tahoe 5.7L 4x4 the check engine light is on and blinks when at an idle (when first started). It idles very rough but runs fairly smooth all other times. I have changed the plugs and found a white residue on the #5&7 plug. This improved the rough idle for a few days. I recently removed the oil cap while the engine was at an idle and found it had a significant vacuum. Once the cap was removed the engine idle became very unstable and it comes close to dieing. I checked the PCV valve and seems ok, it has a slight vacuum at idle and rattles when I shake it. what could be causing this problem?

Answer
Hi
This is JUST a guess from what you say.
I will say up front, you really should go get this checked at a local repair shop your familiar with and trust.
I think you have a faulty lower intake gasket OR a bad head gasket on that side.
The Check Engine light is blinking because it is a POTENTIAL Catalyst damaging fault. if gone on too long unchecked it will damage your catalytic Converter.
It is probally a P0300, misfire code.
I would go get lower intake gasket checked (leaking vacuum into crankcase)May also be the white on plugs (coolant) Check your coolant (antifreeze) level when cold in Radiator, see if it is not low.
If so I would almost bet the lower intake gaskets.
Please, these are JUST suggestions from experience.
Have it checked locally by a repair shop your familiar with for exact diagnosis.
Good Luck
Paul