You really do need a battery. 6 Volt lead-acid type as for a burglar alarm, if you don't want to rebuild an original battery. The AC adapter doesn't supply enough surge current, even the 3-Amp one. Also, those adapters are getting old, and don't always supply a steady 7.5 Volts. I had one jump to 19 Volts recently and burn out my Power CD. I suspect when the Portable is trying to fire up the fluorescent lamp, boot, and spin the floppy, it draws too much current. I've also seen several backlit Portables whose internal 5V power regulator is unstable. This can cause similar symptoms.

A backlit Portable needs the Portable and/or Backlight init/control panels in the system folder. It works without them, but the screen may not be lit correctly.