So those are 25mm tires. You may be able to put 28's on - http://www.bikeradar.com/us/road/new...nounced-41702/ - depending on your year and the tire. Presuming you have disc brakes?

I agree with NYBiker - though - that sounds "off" to me. I don't feel *that* unstable on skinny tires (though 28's and 32's are my preference). Are your bars way below your saddle? Do you feel uncomfortable other times (besides when descending)? How is it on flats/climbs? Is there something about the bike that is making you feel like you don't have control? Can you reach the brakes easily?