Poor or insufficient grounding of intake or sender unit is not a cause since you stated gauge reads high. Poor grounds will cause gauge to read low. You are correct that a improperly functioning instrument voltage regular will also affect the fuel gauge. Good point. But how do you know for sure the fuel gauge has not been affected as much as the temp gauge? Since it is near impossible for an owner to validate fuel gauge accuracy without removing the fuel sender unit, I wouldn’t try to validate the fuel gauge at this time. So it’s back to validating the IVR. The falure modes for this device are that it typically cause the gauges to read too high or not at all. A simple test you could do is to get one of those cheap 12 test light probes and touch the point on each regulator terminals. One side the light should be on constantly. The other terminal it should have a very erratic flashing light. On for a fraction of a second to several seconds and constantly flashing. The case of the IVR MUST be securely grounded. Make sure the mounting screw is really tight and the IVR is secured.
TT