SIM800 APN erases

Good afternoon, I have a problem when connecting to the GPRS network with a cheap SIM800 L I bought on ebay.

After configuring it with the APN of my network provider (do not require a password):

AT+SAPBR=3,1,"Contype","GPRS"
AT+SAPBR=3,1,"APN","internet.nuevatel.com"
AT+SAPBR=3,1,"USER","VIVAINTERNET"
AT+SAPBR=1,1
AT+SAPBR=2,1

and establishing a network connection:

AT+HTTPINIT
: OK
AT+HTTPPARA="URL","**broken link removed**"
: OK
AT+HTTPACTION=0
: OK
: +HTTPACTION: 0,200,126
AT+HTTPREAD
: +HTTPREAD: 126
: {"valorAnalogo":"220","valorDigital":"1","createdAt":{"date":"2016-08-29 16:01:16.000000","timezone_type":3,"timezone":"UTC"}}
: OK
AT+HTTPTERM
: OK

I get sometimes a Good response, (like the one I got above) and sometimes I get no answer or ERROR,

After struggling a little bit I found that the APN gets erased sometimes (leading to this ERROR) and sometimes just remains, giving me a good answer, I check this with the Command:

AT+SAPBR=4,1
: +SAPBR:
: CONTYPE: GPRS
: APN: internet.nuevatel.com // This gets erased sometimes leading to error
: PHONENUM:
: USER:
: PWD:
: RATE: 2

I thought of the source but currently I am using a 5V 2Amp source and the ERROR remains, (less often I have to admit)

Is there anything I should know that Erases the APN and leads me to this problem??

I look forward to any Answer, thank you in advance.

You feed 5v directly to SIM800? IIRC, 5v is too much and causes SIM800 to reset due over-voltage.
I can't now test that with mine SIM, but I've played bit with it. Do you have external antennae connected?