There was a couple of months that were really stressful
There were a couple of months that were really stressful
Which one sounds better? Are they both acceptable?
You might think there is the subject of the sentence, but it's not -- this construct is a special case and here, the subject comes after the verb (reference).
Was is the singular past tense of be, and were is the plural past tense form of be. Since months is actually the subject, you would use were.