Would extremely cold weather [-15 affect stored '88 corvette ecm?

More likely that humidity, age, neglect will affect an ecm- however, make sure that your diagnosis is correct before you go replacin' computers-

It was stored in perfect running condition in Nov, May 1 started & stopped wouldn't keep running, Replaced computer works fine

Heat and cold cause the ECM boards to expand and contract. The solder could crack and not allow a proper connection for the electronic components. In the old days it was cheaper to test and fix the boards, but now it is too labor cost prohibitive to have a tech test and fix it. So test and replace is the norm. The older the boards are, the more susceptible they are to weather variants.