regarding the query that your voice/speech gets bad and that it again becomes normal you have to check few things:
1. you should check the BER/RxQual and FER value.
2. you should the C/I value because the voice quality usually gets worse due to interference.
3. you need to check that if the hopping is enabled or not because then you need to check the MAIO and HSN to identify the faulty DRI
these all reasons usually contribute to bad speech quality. even sometimes your mobile gets faulty and it is actually disrupting the voice quality.
thnx and i hope you may find the good reason of bad quality.