It's the sheer volume of the ice box when compared to your refrigeration unit. It's not the insulation as the 36T's are insulated with many inches of closed cell foam. I've seen it first hand on my 79 as I had to drill through the engine box wall when installing the Sea-Frost system. That wall alone has well over 4" of closed cell foam.
You can try a computer fan to move air around more evenly and it may help cut down on the non-cycling. You may also be low on refrigerant? Another option is to leave it running all week at the dock so your not trying to chill a massive box over a short weekend sail. If your on a mooring not such a good idea.. The sheer size and thermal mass of that ice box takes a while to bring down to temp.
We have a Sea-frost engine driven unit and after 24 hours the ice box is still about 38-39 degrees at the far end, measured opposite the holding plate.
Here's a link to a VERY large photo of the insulation inside the walls of the 36T.