It sounds like the link drops because there are too many retransmissions not going through. Looking at it, I would say that you have many other wifi networks in the area that are interfering with your bridge link. Not much you can do about that.
try :
config t
int d0
packet retries 16 drop-packet
This command will avoid the link to go down if there is an interference problem. Now it will not mean it solves the problem (packets will still be dropped when too much interference) but at least the link will not go down all the time.
