The likely cause, in my opinion, is drowning. The next most likely is some sort of disease. Also, while they can tolerate warmer temperatures briefly, drastic, precipitous changes in temperature should be avoided at all cost. You said it gets from 60-90 degrees - how quickly and frequently does it shift in either direction?

I'm raising 2 pacific chorus froglets. I started out with 5 tadpoles. One died from bloating issues. Another drowned while in the last stages of metamorphosis. I came home one day to find it limp in the water It made me realize that the water level must be extremely shallow to prevent this from happening again. Those couple of days or so from when they pop their arms to the point when their tails disappear are very touchy. The objective is to eliminate the possibility of drowning. That means keeping a water level that is barely enough to keep the morphlings submerged - 1/4 inch or so. It's not the ideal depth - early morphlings which are still more tadpole than froglet will struggle to get used to it, but as long as the water is above their heads, they should be fine. Simply leaving objects and "lifelines" sticking out an otherwise deeper pool will not prevent the occasionally suicidal morphling from finding a way to drown itself.

The two I have now are in a critter cage lined with a 1.5" thick layer of sphagnum moss. As long as the moss is wet, it provides all the moisture that the frogs need without the possibility of drowning. Every week or so, I wring out the moss and rehydrate it with fresh water, removing feces and dead feeders in the process. I probably won't introduce a water dish until they get bigger.