Using MSI Afterburner or some other tool that lets you configure NVIDIA GPUs like this, you have to adjust the VF curve so that each point is higher than the default. That is, you don't set a voltage value, you have to tell the GPU what frequency you want at which voltage step (and there's like 2 dozen plus steps). Though using the OC scanner will do this for you automatically. So yes, in order to "undervolt" the GPU, you have to overclock it, because the GPU runs on a VF curve.
Looking at the what you're saying about the temperature though, the load temp is fine, but the idle temp isn't what I'd consider normal. So we have to look at the following areas:
- Is the GPU clock speed while the computer is "idling" higher than say 300-400MHz? If so, either you have something running in the background or if you have multiple monitors, their refresh rates don't line up. e.g., mixing a 144Hz display with a 60Hz display causes GPUs to idle higher because the have to handle the disjointed refresh rates.
- Does the GPU have a "zero RPM fan" mode? If so, then it'll idle at higher temperatures. However when I see this, they normally idle around 50C unless the above condition is going on. And even then, the fans should kick on well before 65C
- Do you have sufficient airflow through the case?