For old Macs (and aged electronics in general), some machines are best dealt with by removing all power (including PRAM) and letting them sit for a week or two. Others can be revived by total disassembly and storage for a month before carefully inspecting and cleaning during reassembly.
What I'd do with your computer is to pull all power from it and let it sit for a week, at least two or three days. Then hold down the power manager reset switch for 60 seconds before plugging in the power, and waiting another 60 seconds before pressing the power key (unless the machine auto-boots first).
If that doesn't work, I'd completely disassemble it, let it sit for a day or two, then build it up to minimum (no RAM expansion, no Ethernet/modem card, no hard drive) and apply power. If it boots, add the HD. If it still works, add the Ethernet/modem card. If it still works, add the RAM. If it still works, close it up.
That's about all you can do, unless you manage to get ahold of a detailed troubleshooting schematic complete with individual test points and associated good values.
