Where can I buy a 100W or more Magsafe?

i have not heard of this before, i suspect you might have some other type of problem.

The 85W adapter provides 4.6amp as i remember @ 85W. which should be plenty for what you are describing...... other wise people pushing their MBP's hard while burning or reading something from the optical drive would have the same issue.

I would remove your 2nd hard drive and run a stress test on your mac ( a program designed to push it to the max for an extended period of time..... you should have something built into OSX to do this..... or just render something.....or re encode bunch of movies or what ever you can think of to max out your system, if possible keeping an eye out for how hot your system is getting.

Then if no problems, put your 2nd hard drive back in, and do it again. If your 2nd hard drive caddy or power connector however it is hooked up may be to blame, which is a wild guess on my part...... It may also just be getting quite hot with the 2nd drive installed, if too hot the computer will turn it's self off. (your question didn't say if it powered shutdown or just off....... like turning off a light in a room).

Someone else might be able to provide more information as i have not had to deal with this exact issue before.

You can build a DC inverter..... i have one...... i can power car lights or something as small as router with it. However it is quite large since it was built before i was born and designed to power multiple things at once with various options for how much power it produces...so handled without care it would easily blow something up........... so making something to do what your asking would be a fairly large device and quite an undertaking since the power adapter for your mac does more then just produce the power needed for the computer. ( some kind of homemade thing or modification of your current power supply may blow up your computer).