'Grabby' brakes

Yes that sounds like a booster issue. Actually it sounds like a M/C pushrod adjustment issue. Get that thing adjusted right you will have very nice brakes.Its also possible that the mechanism that controls the boost inside that booster is busted. So try lenghtening that p-rod about a 1/32, and see what happens.There is a spec for that, but I dont have it.
It sounds like there no boost, until you have pedaled it quite hard and then the boost comes all in. By lengthening the pushrod, the boost will start sooner and softer.If you cant find a spec. it may take several tries to get it right. The M/C just needs to be unbolted from the booster,slid forward a few inches, and the adjuster is right there.

edit; on the MidlandRoss single diaphragm unit its .915 +/- .005.
This is from the mounting plate (your plate is on there,right?) to the top of the acorn-nut adjuster. The MR unit is the large diameter unit with the band-clamp holding the two halves of the chamber together.