HP networking unstable over fiber connection

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http://serverfault.com – I have a 3com Baseline Switch 2928 that's connected over fiber to a 3com superstack 4500. The fiber is singlemode and has the suitable SFP modules on both switches. The switches (and connected devices) have working networking but it's very unstable. Browsing SMB shares is workable but has lots of 'waiting intervals': sometimes opening a folder takes 20 seconds after which everything is back ok. All devices on the server side have very stable networking, so the issue must be with the baseline 2928 or the fiber SFP module. Are there any logs I can check or other things to see what could be th (HowTos)