Sorry--from what info you've given I can't tell why they are disconnecting after a few seconds. Version? Direct connection? Firewall open? NAT mapped? How many try at once? (if more than 6, try disabling ultrapeer). How many files are being hashed?
Lots of possibilities. I don't know how the code determines when to keep or reject connections, or what it is about your setup that triggers the rejection. Maybe if you gave some specifics someone else can help.