Q: iOS 8 3g service radomly disapears

Hello there,

It sounds like you are unable to consistently conenct to your cellular service on your iPad. I would use the troubleshooting in the following article to help resolve that issue, named:

Get help with the cellular data connection on your iPad (Wi-Fi + Cellular Model)

1. Check for a carrier settings update.
2. Update your iPad with the latest version of iOS.
3. Remove the SIM card, then insert it again. 
4. If your SIM card uses a SIM PIN, tap Settings > Cellular Data and turn SIM PIN off.
5. Tap Settings > General > Reset > Reset Network Settings.
6. If you have another SIM card with a valid cellular data account, remove the SIM card from your iPad and insert the other SIM card. If the cellular data connection works with the other SIM card, contact your carrier to report the issue.
7. Try another location. If the cellular data connection works in another area but not in the first location you tried, contact your carrier to report the issue.
8. Restore your iPad as new.

 

If you still can't connect to or use a cellular data network, contact your carrier, make a reservation at a Genius Bar, or contact AppleCare for more help. 

Thank you for using Apple Support Communities.

Take care,

Sterling