Although I really don't think that shielded Cat5 is necessary do you have the shields all tied together and grounded at 1(and only 1) point?
I really believe it's coming from the CO.
Sometimes it's hard to pick it up on an slt or buttset as there generally more forgiving then a switch or ksu which could actually be amplifying
the problem.
who is the CO provider?
You may have to just keep reporting it until you get a Tech. who's willing to check the line current and loop resistance coming from the CO. They can also check for continuity between pairs. The real problem may be getting someone to do this.
If you complain enough they will probably change the pairs to the NID or D-marc.
Good luck.