1999 Mercury Sable electrical problem?

My car kept losing its charge on the battery. I thought maybe it was the alternator, so I drove it down the highway at 65 for 1 1/2 hours. I held the charge fine for a few days. Then I got in it after sitting over night and it acted like the battery was dying. The wipers were in the off position but were running as if on. I messed with the wiper switch while turning the key and it started. It ran for a few more days and then stopped. I tested the battery and it says it is good. If I jump it, the car turns over but doesn't catch. As soon a I take it off the jump it's dead again. Everything else electrical seems to be working fine.