Ok, so this is the story my wife tells me. She's driving home and takes the off-ramp to our house and at the top of the ramp she stops at the stop sign. When she rolled away from the stop sign the car started to stutter and hesitate. She gave it a bit of gas, figuring it would help to get the rpms up. When she got up to speed it smoothed out. Two or three blocks later she made another turn and got the same thing. Maybe a mile later she made the turn into our neighborhood and the car really started to act up, chugging and surging the last 2-3 blocks to our house. When she pulled it into the garage, she quick parked it and shut it off. Immediately after the first "stutter" the MIL started to flash and was on solid by the time she parked. She also shut off the A/C and the radio to see if she could hear anything and didn't notice any significant noises. A few seconds after she parked the car she tried to start it and it would crank, but not fire. ~2 hours later I tried to start it and it wouldn't even crank.

Code reader indicates a P0300 (multiple cylinder misfire), nothing else. I've removed the serpentine belt and all accessory items (A/C, water pump, etc.) turn freely. Engine will still not crank with the belt off. The engine is using more oil recently (1.5 qts since last oil change ~4000 miles ago), but is still full. I also had to add a bit of coolant to the reservoir at that oil change and the reservoir is currently empty. I couldn't see any coolant in the radiator when I removed the cap, so I stuck a "dipstick" in and found the level is about 6" below the radiator cap. I asked my wife and she said that she was a bit flustered, but she didn't see any low coolant or over temp lights and that the temp gauge looked "normal" when she parked the car. Both the oil and the coolant look "clean".

Any ideas on what the problem could be or how to troubleshoot this?

Can you turn the engine if you put a wrench on the bolt for the harmonic balancer and try to turn it that way? It's possible that you have an intake failure and coolant has gotten into the cylinders and hydro-locked the engine. If it doesn't turn, remove all of the spark plugs, and see if the engine spins. If so, continue until you no longer see water/coolant being blown out, and drain the oil immediately. You'll find more information here, in techinfo.
http://www.pontiacbonnevilleclub.com/forum/viewtopic.php?f=53&t=1061
http://www.pontiacbonnevilleclub.com/forum/viewtopic.php?f=53&t=3591

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*Gone, but not forgotten* - Black 2000 SSEi, HIR Headlights, Angel Eyes fogs, 3rd brake light overlay, hi-flo cat, 180 degree thermostat, HS 1.9 rockers, LSx yellow springs, Intense FWI, PCM, shift kit, push rods, and 3.4 Pulley. ZZP Power Log.

I was thinking that same thing last night and that appears to be exactly what happened. The harmonic balancer had a bit of play in it when I tried to move it with a wrench. I followed up by pulling the plugs and trying to crank it over, which it did just fine. After that, I looked in the engine compartment and saw where the coolant had sprayed from the cylinders.

When you say intake failure are you referring to the gasket? I haven't checked out the links you sent yet, but I will. Thanks for the help.

*** EDIT ***

Now I understand Intake failure. Any idea what the parts would cost to fix this, including the lower intake manifold gasket? And a guess on what it might cost if I take it to a shop? Nothing too fancy, just to the closest $100 or so. Thanks again.

Guestimate of $200-$300 DIY. $600 - $800 at a shop. Be sure to get the metal framed LIM gaskets, either GM of Felpro. Also, if you haven't done so already, drain the oil out of the engine, get some cheap oil and put that in, and crank the engine again. The longer the lower bearings sit in the coolant, the more likely you will have a bearing failure after the repair.

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*Gone, but not forgotten* - Black 2000 SSEi, HIR Headlights, Angel Eyes fogs, 3rd brake light overlay, hi-flo cat, 180 degree thermostat, HS 1.9 rockers, LSx yellow springs, Intense FWI, PCM, shift kit, push rods, and 3.4 Pulley. ZZP Power Log.