I have faced similar issue with my 8900.. but mine was switching from upper case EDGE to lower case "edge".. I was on 184.108.40.206 but after I upgraded to 220.127.116.11 I have never faced the same issue again... it could be a coincidence but I highly doubt this behavior is due to the OS.. it is more likely a carrier issue.
You may want to upgrade to 18.104.22.168 and check if you face the same issue. .174 although is a beta but it has been stable till now.
__________________ Regards|Raja BlackBerry FAQ Nokia 2100 > Nokia 6100 > Samsung t319 > Nokia 5300 > Blackberry 8320 > Blackberry 8900 > iPhone 3GS > iPhone 4