Serpentine belt tensioner is jumping and acting weird

The belt tensioner is acting strangely – can anyone help diagnose this problem?

Engine: Crate Magnum 380/360 with a serpentine set-up from a mid-90s Jeep. Everything was fine before I changed the belt set-up: I deleted the power steering pump and added a dummy AC idler on top to get the belt geometry right, and fired it up for the first time yesterday.

It took some experimenting to find the right length belt but I finally found one that keeps the tensioner in about in the middle.

The belt tension “feels” right in that I cannot quite turn it over 180 degrees.

Symptoms:
It is unclear exactly what is happening but almost acts like there is a hitch in the belt or something that is momentarily (and randomly) grabbing the tensioner pulley as it goes by and is jerking it inward (toward “looser”) and then letting it snap back. It is quite audible and also you can see the belt loose tension around the crank pulley momentarily.

First thing I did was run the engine with no belt for a few moments and it sounds like it should.

So I went out and bought a new tensioner and put it on. Same problem.

Next I put added washers under the dummy AC idler to add a little bit more tension. Same problem.

Any thoughts?

Thanks in advance!