Rythmic Changing of RPMs when stopped and braking

So my truck has been having an issue with this for a while, however sometimes it goes away and then sometimes it's there. The problem initially is that when I would come to a stop and only when the brake was pushed in the engine would rhythmically change rpms (fast, slow, fast slow) but when I whole take my foot off it would stop and the engine would idle normally. However recently it's gotten worse and the rhythmic pulsing of the engine now happens at a higher rate and when the brake is pushed in and also now when the brake isn't pushed in but a at a slower rate.

Thanks, Eli