900mhz Wireless Internet Probs

I have a Rohn 64 tower with a 900mhz yagi at 69 feet. direct line of sight to the tower 2.5 miles. Nothing in the way, we went up on a boom and confirmed this. We are constantly losing signal or dropping to dial up speeds. The few times it works it works great. The ISP uses Alvarion equip. Any ideas?

If it helps, I have 95 feet of cable from antenna to modem....