Voltage limiter, 69 Dart

The 5v will not look like 5v on a digital multimeter.

It will look like your meter is going haywire because you are measuring a square wave.

If you suspect your IVR, replace it with one from ,R-T engineering. Pricy but the benefits out way the cost

RTE limiter - rte

You can test it with a test lamp, look for a rise and fall of the lamp brightness.