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Disconnecting ground is, in most cases, not a good idea. To begin with why:


Instead of a "perfect" ground connection, your Attiny ground now runs through Q4. That's a recipe for noise and irregular behaviour. You seem to be using the ADC - not a good idea
The devices on the I²C bus will still define a potential for your I²C lines – disconnecting ground here is an especially bad idea


So, you'd really want to switch VDD in this situation. Of course, the decoupling capacitor will allow your attiny to run quite an undefined while after deconnecting.

So, really, reset your device usnig your device's reset. If your device doesn't have a reset pin, get a different one. Or one with a watchdog that you can reliably trigger.