Was dusty power supply causing pc restarts and freezes?

It sounds like you probably had an intermittent short circuit that interrupted the power going to the rest of the computer. As long as the shorts didn't end up sending a power surge down to the motherboard or other components, then they're probably fine. Removing the dust probably helped, but there's a good chance that those short circuits may have damaged components inside the power supply. It would be a good idea to go get a new power supply, because if there was any internal damage, you could end up damaging other components in the computer. When you get a new one, make sure it has the same or higher wattage, the same type of connection to the motherboard, and at least as many other connectors as the old one.