iChat forgetting specific buddy. Not allowing add. Must delete .plist

[DaveHuber]

I have iChat on 2 macs.

One uses a .Mac account. The other is an AIM account.
frequently, one of the macs will not show the other's buddy name. I cannot add it manually. It blinks for about 4 seconds and drops off the buddy list.
The only "cure" is to delete ichat.plist . I would prefer to fix, rather than treat the problem.

once running, all is well. Video and screen sharing go fine.

Any suggestions as to what I am doing or should be doing to remedy this problem?

Both macs run latest leopard, and are up to date.

Thanks