Replaced hard drive, but still freezes and trouble starting?

With it not restarting, it sounds like a heat issues. Open it up and clean the fans. For a better result, open it and remove the logic board and clean out the heat sink vent ports and replace the thermal paste. Warning: this is a difficult procedure and you really have to pay attention to get all the cable out from under the board when replacing it. I did one last week and had to take the logic board out twice due to a heat sensor cable getting under the board. I also take pictures as I go to augment the guides. Also ESD protocols must be followed.

Here's how to remove the board: PowerBook G4 Aluminum 15" 1-1.5 GHz Logic Board Replacement

Here's the guide on replacing thermal paste: How to Apply Thermal Paste