My Pomeranian collapses abruptly...?

I have a 9 year old male pomeranian. For years, he's had what appears to be 'allergies', he's often congested and sneezing, and it's usually far worse during the Spring. Just last night, he was running around and playing then all of a sudden it was as if he lost muscle strength in all of his legs. He fell abruptly and stayed limp for about thirty seconds before recovering, very slowly. He's fine now, but this has happened about 4-5 times in his life. We took him to the vet once, and she said it was 'probably' from exhaustion, but I just want to get a few more opinions before I take him again.