Try this.

Get ready to do a stop from say 40MPH shift to Neutral and keep the rpms up to, say 1200 or so and then do the braking. See if having a little more rpm (Oil pressure from the pump) helps the issue.

If this does not help then ABS is probably having issues.

An issue with the ABS should show a code or at least the light should come on to alert you to an issue.

If the extra RPM's stop the problem then I would suspect the booster has issues.

Lower oil pressure with a booster that starting to fail can result in some hinky things happening when braking.

Let us know

Robyn