I have been having the same problem with my pismo 500. At first the machine would die when I picked it up, but usually only if I used just one hand near the battery/dvd part of the laptop, causing the entire laptop to flex at the middle point. I had also noticed that it would boot if I pressed hard on the keyboard when trying to boot.
After taking the computer apart a couple of times (fun fun, thanks go to http://homepage.mac.com/sysop/PhotoAlbum7.html
), it hit upon me that the connection between the power supply card and the motherboard must be the problem.
It seems that the power supply card was designed to have a screw near its connection point to hold it in place, but apple did not attach a screw there, since the motherboard below has circuits running in that location.
I managed to fix the problem by cutting a paper clip apart and attaching short metal pieces to the some of the screw points to form a lever which would keep the power supply card pushing against the motherboard to ensure a good connection. (screw 1 of image 2 and screws 1 & 2 in image 13 of the slideshow).
i have posted a picture of the modification:
It seems to be working for the time being. let me know if it works for you.
jasperv at mitdotedu