So if I read your comment correct - you can overcome your problem with two scenarios:
A) Changing new/old sim card
B) Network provider can kill the last active PDP context (last active session), and you can re-attach PDP context
Have you checked if new firmware update exists for your Q2403a ?
Also check the trouble shooting section of your wavecom user guide, maybe its equal to this version:
http://www.rfsolutions.co.uk/acatalo...User_Guide.pdf
Notice that after a PDP context attach and your modem has been idle for X minutes (maybe this is a setting somewhere in modem) the PDP context
moves to "idle state". And only when data is ready for transmission it will force PDP context back to active state. Now maybe something in this process goes wrong (wavecom modem problem or PC/driver problem).
If problem persists - try another GPRS modem (e.g a mobile phone with GPRS and modem access point cabability + bluetooth/IR connection to PC).