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A properly functioning 3750X stack, functional behavior shouldn't vary just depending on which stack member you connect to.
What IOS version is being used?
Is there any difference at the configuration between this problem ports and ports that works well? Does another devices works well at this ports? Is there any errors at this ports?
Both your Gi3/0/21 and Gi3/0/24 ports have hardcoded speed and duplex. Can you please configure both to "auto" and see if there is any progress.
Well, the last test that I can to offer is to put another device (not a phone) to this port and look how good it works. If there is problems as well than there could be damaged ports. If not than it's better to contact TAC if you (or your client) has a SmartNet contract.