PE 5548 dropping connections

I have a setup like that:

Allied Telesis x908 (10Gb GBIC) --> (10Gb GBIC port te1/0/1 ) standalone PE 5548 --> (10Gb GBIC port te1/0/2 )  --> 2 member stack PE 5548 (10Gb GBIC port te1/0/1 )

Even the first standalone switch give me odd hickups, which show themselves on the connected client, as ie lost connectivity, no routing etc

It last very short time (seconds), but enough to ie kill application that runs across the network

Can not see anything in logs (syslog)

Any idea how to tackle it?

Seb