Do you have a custom recovery? If so nandroid (backup everything) and factory wipe. If not, it's worth trying anyway just less convenient without root or cus. recovery.
Basically there is little reason why your connection should get worse expect hardware failure or network problems, this could could be as simple as a bad contact with the sim, but it's worth ruling out software issues first as they're normally fairly simple (and free). It's also worth trying a different network if you can. It'll help if we can start ruling more things out.