Ignition Coils and Antifreeze/Coolant

You're losing coolant every month? Do you have an external leak? A cooling system pressure test may help find the source of the leak. The shop can use their emissions (smog) sniffer to test for any hydrocarbons in your coolant reservoir. They don't submerge it but it can detect exhaust hovering it above the coolant level Multiple cylinder misfire can be caused by a number of things. Have they inspected for air/vacuum leaks from hoses or around your intake manifold and plenum? Are they driving your Sentra with the scan tool and watching the real time misfire data? Is your car covered under the Cam and crank sensor recall? Has that been performed? Are there any drivability symptoms like surging or bucking or just a loss of power? Have they made sure your ECM software is up to date?

I have a similar problem with my 1997 Crown Vic- but I know why it's happening on mine. The intake manifold is a faulty design and it leaks coolant out one corner, which then drips down into the spark plug shafts and causes one side (4 cylinders) to misfire. BUT it only happens while driving (under pressure) but you can see the trails of where it's running down the block. It doesn't leak onto the ground while idling because it burns off on the block- but it's a minor leak as well, and I'm not going through nearly as much coolant as you. It's odd that your dealer can't figure out where your coolant is going- seems like there aren't many things that could be happening to it besides an outright leak. It might only leak while driving like mine, I would say try to have them test it while it's driving.