could be some kind of dns issue as well
[...]
cleared all caches on our cache-only nameservers; not using forwarders; but still always get the same results back
That's exactly how the Akamai DNS is supposed to work. You'll always get different results from different AS's as the Akamai DNS tries to find the nearest Akamai cache cluster using some BGP + DNS magic.
See: http://web.archive.org/web/20030302122757/http://www.cs.washington.edu/h omes/ratul/akamai.html
cheers, - Fabian