Hey guys, wonderfull work all arround

here's my somewhat weird issue.
I'm running a store with some 7500 customers and have a very weird problem.
The whole admin works fine with the exception of Admin Customers. When I want to check some customers details and check Customers the queries take for ages - the last time I checked (just now) it took 8 minutes to open the customers screen, and when I try to find a specific customers it takes even longer.

On the other hand when I open any other admin subpage there are no problems, including orders with some 7K entries

I've tried optimizing, repairing tables in mySQL and in the shop, to no awail whatsoever.

I've had this problem since ZenCart 1.2.4, when updated to 1.3.0 it didn't hellp at all, now on 1.3.7 the problem is still here

Does anybody have or has had any similar problems, and any ideas how to fix this?

thx

server info:
Server OS: Linux 2.6.9-023stab043.1-enterprise Database: MySQL 4.1.21-standard
PHP Version: 4.4.4 (Zend: 1.3.0)
HTTP Server: Apache/1.3.36 (Unix) mod_auth_passthrough/1.8 mod_log_bytes/1.2 mod_bwlimited/1.4 PHP/4.4.4 FrontPage/5.0.2.2635.SR1.2 mod_ssl/2.8.27 OpenSSL/0.9.7f