That's the usual power supply failure. Is the same thing happening without a battery?
Are you comfortable with taking it apart to wiggle all the connections and seeing if the CPU heatsink/module is in place?
Bob
This 7400 unit running Win 2000 and WinME is 366hrz with 256 ram cycles off and on on its own when power cord is attached. The Win 2000 is the default op system. After power up it may run 5 mins or so then shut itself off and then after a few mins idle will restart itself without me moving the on off switch. At first i thought i had a virus as upon startup the mouse would go berserk and be visible only around the extreme outer edge of the screen. The mouse will freeze sometimes when the unit boots itself up. The screen flucuates brightness after and during the boot sequence. I can sometimes perform some work after it has completely booted but just opening a folder can prompt it to shut down or it is coincidental to its short boot period. When mouse is at outer edge it cannot be used to open folder or programs.

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