New Temp and Fuel Gauge Problem

Only the temp and fuel gauge use the IVR. My OE mechanical has a capacitor attached to its output. Most likely to smooth the pulsed DC from the mechanical IVR. Perhaps you left it in or out? and the electronic IVR is needing or not needing it.

My guess is that you have a bad IVR.

The mechanical ones have worked fine for 50+ years new is not always better.

When you say you replaced the fuel and temp gauges is that with OE gauges or aftermarket? I recall seeing that the aftermarket have specific power and sending unit requirements.