Worried about Sam - sneezing followed by excessive drooling

This has happened twice in the last two weeks: Sam has a sneezing fit, about 10 sneezes in a minute or so, then drools nonstop for two hours. He's normally has a pretty dry mouth. The first time, we were watching TV, almost bedtime, and he jumped up, did the sneezing, then the drooling started. I checked his mouth to make sure something wasn't stuck in there, and gave him a cookie and got him to drink water. He seemed fine other than the drooling, we eventually fell asleep, and he was fine in the morning.

It happened again on Saturday. We were getting ready for bed, about 10 minutes earlier the dogs had gone into our room to go to bed, and Neil and I were shutting down our computers. Sam ran out and downstairs. Neil followed and let him out, he said Sam just walked around sneezing. We went to bed, and we had to put a towel under Sam because he was soaking the sheets he was drooling so much. Again, fine in the morning.

I'm thinking that something is getting in his nose (dust or pollen) to cause this. Any other ideas?