Thanks for your quick responses. I thought my problem was unique, but searching the boards has also turned up a lot of insights.

It looks like the previous owner has not been treating the cooling system that well, and the radiator is getting old anyway, so I decided it would probably be best just to go ahead and replace it now, to get some peace of mind on my trip. I found a good price from www.radiator.com and they're willing to deliver in one day, so I'll install the new radiator tomorrow.

Thanks again!