ATT iPhone 5 dropping data after LTE

After being on an LTE network, my phone will not reconnect to a 3g network.

I have to turn it off and back on and it connects right away.

They've sent me a new SIM..didn't work. It's been a month of "troubleshooting" by AT&T...no resolution.

Any one have similar issue?

I recently traveled and this seems to happen after connecting to LTE in a few different states..CO/CA/OK

TIA