att s3 on ST with issues

ok, sgh-i747. fresh NJ1 rooted flash done yesterday. using phone on straight talk with att sim. supposedly lte capable. at work i can pick up 2-3 bars of lte no prob. runs great. but this morning the phone automatically switched from lte/gsm auto. to wcdma mode on its own. it did this three times in a row inside ten minutes. then all good most of the day. however this afternoon it did it again twice. sticker on phone say i727. phone info states the same. so what would cause a gsm phone and gsm simcard to suddenly go wcdma for no reason? i do suspect a bad sim. ive heen having connection issues as well. at home signal isnt the greatest but on att i always maintained 1-2 bars of 4g. using the straight talk sim it will connect to 4g for a few minutes maybe, and then drop to edge. if i am not on lte, it will not stay on 4g. jumps to edge. its not the phone i dont think. this morning i put my sim in gf phone,same exact phone. entered same exact apn settings, rebooted, and waited. after a minute or two it dropped to g then to edge. her att sim in her phone usually maintains a steady 4g at 1-2 bars. never drops. her sim in my phone works flawless. same thing. steady 4g at 1-2 bars. how is that? doesnt the phones radio determine signal strength and what network type to use? it seems my thinking is wrong. everyone says sim cards have nothing to do with signal but after seeing my ST sim react the same in someone elses phone, i gotta think different. my settings are the most common ones

name: Straight Talk
apn: tfdata
no proxy
port: 80
(i did make an apn with no port set as well. makes no difference to me)
mmsc: http:/ / (no space) mms-tf(dot)net
mms proxy: mms3(dot)tracfone(dot)com
mms port: 80
apn type: default,supl,mms,internet,hipri

i made a few different apns with different type combos. and one with no apn type. each time tho i deleted the apn, then rebooted, then entered settings, rebooted again.

would all the rebooting and different apn settings and messing around mess up a sim card? ive had this card about two weeks. seemed to work really good for a few days. and of course tech support is no help. also, was trying to research ST sim cards to see if some were better or some problematic. found nothing for mine

TF64PSIMC4DD

any help or ideas?