The resistor is only to protect the coil from overheating, especially when the ignition is on but the engine not running. Volvo used a high resistance coil, which does not need an external resistor. But if there was a resistor, then it was often removed when electronic ignition is installed as the controller already has current limiting.

An old coil can develop problems when hot (no matter if external resistor or not). An electronic ignition can also develop problems when hot. If you replace the coil, check if you also need to fit a resistor for the new one. To check the electronic ignition, try relocating the box to outside the engine bay as a test. Or else just move it as far away from the engine as possible and leave the bonnet open so that the controller stays cool but the coil and distributor is still close to the engine and will heat up.

Hard starting when hot on FI cars is usually because the fuel system does not hold pressure during heat soak, causing vapour lock. There are 2 or 3 components that can cause the problem, the easiest to find is a leaking injector. (But you say you have confirmed weak spark when hot as the possible problem, so I would check that out first.)