Emails coming through sporadically

[glittergirlnz]

Hello,

Ive got 4 blackberry curve 8520's on 3 different BES servers that, for some reason, their emails come through all at once but only every 2-4 hours.
It almost looks like the device has fallen off the carriers network and reconnected and pushed all the emails through, but the devices all have full signal even when the emails arent coming through.
Forcing the device to reconcile doesnt work.
These are on 3 different BES servers Ive checked the policies and theres nothing unusal there.
With two of them I swap the devices and the SIM cards and it fixed it, but the other two it didnt. Ive removed them from the server - same issue.
I also have an 8310 that does the same thing.

Has anyone seen this before??