Nuvi 660 and Bluetooth 'breaking up'


Well.. since I got my 660 back I've noticed that the BT phone is struggling a bit.

V4.60 BT V3.1
Phone: Blackberry 7520

It worked perfectly on the previous 660 that was running V4.50 and BT V2.4
(I had backdated to 2.4 for known blackberry connect issues. v3.1 was reported to have fixed this so I let it upgrade from v3.0 to v3.1)

When using the BT phone, it would be ok for the first minute or two.
Then it would start to break up. I can't say its a 'connection' issue as it never disconnects.
When I 'switch to phone' in the middle of the call, it sounds fine thru the speaker phone on the berry. Switch the call back to the nuvi? Its sounds 'broken up' again. This confirms its not the phone's cell connection.
To me..Its as if the processor is struggling to push the audio.

Has anybody experienced this?

Considering the no-backdating with the 7xx series, its it possible to backdate to 4.50 and/or BT2.4 without nuking the unit?

My first step would be to backdate to BT V2.4 If there is no improvement, I'd like to backdate the firmware to 4.50

Thoughts? Experience? Advice?
Bueller? Bueller? wink

Nuvi 350 Born Oct 07 - Nuvi 660 Unit #2 (re)Born Sept 08 - Nuvi 360(Gift to 'the chick' yet maintained by myself) Born July 08