see whether u wanna try leak rom or the more stable rom

usually, a huge version gap means there is a major update on the android version for example 4.1.2 to 4.2.2
in this case, my guess is 6.8 is the 422 leak rom while the 5.1 is 412 rom

baseband not refer to telco freq, is it a firmware that control your phone radio(Not FM radio). u can use other countries baseband(eg UK) here in malaysia.

for updated base, since wanam rom is based on samsung official/leak rom, the updated base refer to the newer samsung rom used as his rom 'base'

Do a nandroid backup and change back to your old ROM for a while. Then belasah kaw2 try games, internet and all.. if all ok means software (probably ROM issue).

If you go back to old ROM but still getting auto shutdown and restart. It is highly a battery issue and need to be changed.

Taiwan is getting below for 4.1.2
Country/Carrier Date Version PDA CSC Kies
Taiwan  2013 June          4.1.2  I9300ZSEMF1  I9300OZSEMF1 
Taiwan  2013 March  4.1.2  I9300ZSEMC1  I9300OZSEMC1 
Taiwan  2013 February  4.1.2  I9300ZSEMB1  I9300OZSEMB1

We may get
Country/Carrier Date Version PDA CSC Kies
Mlaysia      2013 June          4.1.2  I9300XXEMF2  I9300OLBEMF2
Malaysia  2013 March  4.1.2  I9300XXEMC2  I9300OLBEMC2 
Malaysia  2013 January  4.1.2  [B]I9300XXEMA2  I9300OLBEMA1