I too was using a CDMA s3, and i started experiencing this issue about a month ago here in Miami FL
i saw the thread posted above, and optimistically gave it a shot
Spoke to what seemed to be a knowledgeable rep, told her about my issues and told her exactly what i wanted to be done, the refresh of my account and signal, it was done by her words, she finally told me to simply call *228 and after my phone automatically re-programmed it would restart and it would be a night and day difference...she assured me it would solve my issues and if by any chance it didnt, to call back from another phone and try and do it manually. After the *228 'reprogramming' my phone turned on to the same exact issue, i was so frustrated i didnt even bother trying again, but i guess the possibility was there, as per members posting on that thread, and by the 'manual' option she mentioned. Anywho my s3 broke, and i wound up grabbing the s4, speeds are better everywhere i've been so far (when compared to my old CDMA s3 speeds) and the signal has been stable, ive only had it jump out of 4G once, in my room, then again my room has bad signal.
Im not telling you to go out and grab an s4 as it will be the only way to solve your problems or any GSM phone for that matter, all i know is in my area nothing worked so i opted for something that was guaranteed to improve my situation.
My guess is the're still tweaking things with the network since MetroPCS is obviously going GSM, i assume CDMA should go back to normal at some point before their shut down in 2015, was i willing to wait for that day, and not face the fact that i'd be going GSM sooner or later?(not that i wasnt going to get the s4 regardless of these CDMA issues) Not really.
Give it a try and post your feedback on that thread, its worked for some and you might get lucky.