I'm sorry to learn about the problems you're experiencing with your mobile signal later, I'd recommend taking a look at our [Signal Checker] to see for low signal areas.
I'd also recommend performing a manual network search, if you're not sure how to do this let me know what handset you've got and I'll search for instructions. If this doesn't work, would you be able to try your SIM in a different handset? To rule out the handset as a potential issue.
Thanks for taking the time to get back to us, as you've checked the network side of things I'd like to double check the hardware is operating as it should be. Also, is your phones operating system up to date?
Would you be able to turn the device off, remove the SIM card, give the contacts a good wipe and re-insert? This could be why you're intermittently losing service and trying this would let us know.