98 Chevy 1500 5.7 PO305 code

[jmrich56]

I have had a PO305 code come and go for about 6 mo. I realized early on that the fault only happened if I let the engine idle 4-5 min. on a cold start. Now it's time to have a smog test and code is more frequent now. The engine does sound and feel like there is a miss @ idle, but it does not sound anything like a dead cylinder miss, just a noticeable dip in what I hear and a slight vibration in the steering wheel. Now some checks i have done:
compression 155 psi ( i did not check other cylinders)
fuel pressure 60 psi engine off / 55 psi running
#5 spark plug has less than 10k miles and appears normal compared to others
swapped #5 plug with #1 - no change in code
replaced #5 plug - no change
replaced #5 wire - no change
replaced spider box recently - not AC Delco but Autozone GP Sorenson
visual check on distributor cap / rotor - appears normal
air filter / fuel filter / pcv valve - new
check for vacuum leaks w/ carb. cleaner spray @ gaskets - none

PO 305 is the only code thrown by computer
truck / engine has 190,000 miles
Performed a valve job @ 125K.
Accelerates great after spider box R&R
Engine does use about 1 1/2 qts oil between change @ 5K
No noticeable smoke or smelly fumes.
Except for the slight rough idle and the code, engine responds well.
I use an Actron OBD II scanner that has freeze frame capabilities.
Short Term & Long Term Fuel Trims #3 & #4 are wacky.
#3 Short Term 80.4%
#3 Long Term -54.6%
#4 Short Term -47.9%
#4 Long Term 28.9%

My next move will be to remove the valve cover and check #5 valve springs.
With so many of these trucks having been on the road, hoping someone else
has endured this and figured it out.