I searched the archives and couldn't find anything similar to this...

A couple days ago I was cruising at about 65mph on the freeway and it started feel like the M30 engine was not firing on all 6 cyl. it started getting worse so I pulled off on the next exit.
As it slowed down it started to run really rough and barely stayed running, I managed to ease it into a parking lot and turned it off. I let it sit for a while then checked the vitals inside.

The oil looked fine, a little low, but no coolant visable.

The coolant expansion tank was empty, so I opened the tank cap and very slowly let the high pressure air out. Eventually the coolant returned to the tank from the system after it was depressurized, it seemed to be very near what level it was last time I checked it.

Then, I tried to start it up, but no crank. Oh-no! Fluid lock? Coolant in the cylinders...? Figured maybe a headgasket, but I don't see the coolant in the oil or vice versa. Well, I left it there and got a ride. Later that night it was towed home.

Finally, got a chance to look at it again last night. It turned over and started just fine...but after running for a bit I could tell something isn't right. Stepped on the gas and it seems to try to rev, but just bogs down like its flooding out.
Absolutely no power. It started running rough, so turned it off. No time to check anything else.

The last thing I did to the engine was put new bosch spark plugs it last weekend and it was running smooth as ever all week long. (gap: 0.7mm)

Anyone know whats wrong? or what do I need to check next? I'll have some time this weekend to look into it more. For now, I have to drive my gas guzzling Chev truck.