as described, the transport from store to home is unlikely to be a factor. >12 months at 80°F at costco may start getting into the range to start wondering.
three other questions that may help shed light:
how long do you let the wine stand after removing it from storage before drinking (just long enough to find the corkscrew, a week?)
how long are the wines being stored by you prior to drinking? (a couple weeks or years later)
can you define the frequency of the temperature cycling? (1/day, 2x/hour, 1/week)?