My bet is that your DOD (Displacement on Demand) has gone bad. How the DOD works is there are oil passages that collapse the #1,4,6 & 7 Cylinder lifters, what happens is the lifters wear and they get jammed and will not release. I am not sure how or why, but that is what happened to me. Before this happens, usually you have a problem with one of those 4 cylinders (1,4,6,7) will keep fowling out the spark plug with oil.
The bad news is 99% of the Mechanics will tell you that your engine will need to have the intake, heads removed and replace the lifters. which is true, if you do it as per the book. However there is an easier way to fix it.
I did this and it worked perfect.
1) I was having #1 fouling issues, every 6 months I would have to change the #1 spark plug,
2) When replacing the spark plug no longer worked
3) I did a compression test, compression seemed fine, #1 was a little lower, something like 130PSI, The others were around 150PSI
4) Removed the (drivers side) valve colver, and discovered that the intake lifter was collapsed, I could wiggle the rocker.
5) Removed the intake, and then used this guy's tool worked like a champ, I broke my intake just as he showed,
6) I was not going to drive it so I sent my ECM to
LT1 Swap and for $under $100 I sent him my ECM, he disabled DOD, and sent it back to me.
All in all I think I was around $400 in parts for everything, the tool, the ECM DOD delete, the intake gaskets, and the special tools I did not have to into the engine.
If you have a mechanic fix it, expect to pay $2500-$3000 as they will want to remove the entire top of the engine, replace everything, which is not a bad thing, but it seems much for a vehicle that is probably worth $3000.