Me sucede algo parecido.
Funciona perfectamente en 3.5G pero de repente pasa a 3G y:
From * icmp_seq=287 Destination Host Unreachable
...
64 bytes from 8.8.8.8: icmp_seq=1 ttl=39 time=289203 ms
64 bytes from 8.8.8.8: icmp_seq=3 ttl=39 time=287205 ms
64 bytes from 8.8.8.8: icmp_seq=2 ttl=39 time=288205 ms
64 bytes from 8.8.8.8: icmp_seq=4 ttl=39 time=286205 ms
64 bytes from 8.8.8.8: icmp_seq=6 ttl=39 time=284205 ms
64 bytes from 8.8.8.8: icmp_seq=7 ttl=39 time=283209 ms
64 bytes from 8.8.8.8: icmp_seq=5 ttl=39 time=285209 ms
64 bytes from 8.8.8.8: icmp_seq=12 ttl=39 time=278216 ms
64 bytes from 8.8.8.8: icmp_seq=11 ttl=39 time=279216 ms
64 bytes from 8.8.8.8: icmp_seq=10 ttl=39 time=280219 ms
64 bytes from 8.8.8.8: icmp_seq=15 ttl=39 time=275221 ms
64 bytes from 8.8.8.8: icmp_seq=14 ttl=39 time=276221 ms
64 bytes from 8.8.8.8: icmp_seq=13 ttl=39 time=277221 ms
64 bytes from 8.8.8.8: icmp_seq=16 ttl=39 time=274225 ms
64 bytes from 8.8.8.8: icmp_seq=9 ttl=39 time=281225 ms
64 bytes from 8.8.8.8: icmp_seq=8 ttl=39 time=282225 ms
64 bytes from 8.8.8.8: icmp_seq=18 ttl=39 time=272227 ms
64 bytes from 8.8.8.8: icmp_seq=17 ttl=39 time=273230 ms
64 bytes from 8.8.8.8: icmp_seq=19 ttl=39 time=271230 ms
64 bytes from 8.8.8.8: icmp_seq=20 ttl=39 time=270231 ms
64 bytes from 8.8.8.8: icmp_seq=23 ttl=39 time=267231 ms
64 bytes from 8.8.8.8: icmp_seq=22 ttl=39 time=268235 ms
64 bytes from 8.8.8.8: icmp_seq=21 ttl=39 time=269236 ms
64 bytes from 8.8.8.8: icmp_seq=25 ttl=39 time=265247 ms
64 bytes from 8.8.8.8: icmp_seq=24 ttl=39 time=266247 ms
64 bytes from 8.8.8.8: icmp_seq=29 ttl=39 time=261251 ms
...
64 bytes from 8.8.8.8: icmp_seq=189 ttl=39 time=101521 ms
64 bytes from 8.8.8.8: icmp_seq=192 ttl=39 time=98522 ms
64 bytes from 8.8.8.8: icmp_seq=194 ttl=39 time=96522 ms
64 bytes from 8.8.8.8: icmp_seq=195 ttl=39 time=95522 ms
64 bytes from 8.8.8.8: icmp_seq=191 ttl=39 time=99525 ms
64 bytes from 8.8.8.8: icmp_seq=193 ttl=39 time=97525 ms
64 bytes from 8.8.8.8: icmp_seq=199 ttl=39 time=91526 ms
64 bytes from 8.8.8.8: icmp_seq=196 ttl=39 time=94526 ms
64 bytes from 8.8.8.8: icmp_seq=197 ttl=39 time=93526 ms
64 bytes from 8.8.8.8: icmp_seq=198 ttl=39 time=92527 ms
64 bytes from 8.8.8.8: icmp_seq=200 ttl=39 time=90527 ms
64 bytes from 8.8.8.8: icmp_seq=201 ttl=39 time=89532 ms
64 bytes from 8.8.8.8: icmp_seq=202 ttl=39 time=88532 ms
64 bytes from 8.8.8.8: icmp_seq=204 ttl=39 time=86535 ms
64 bytes from 8.8.8.8: icmp_seq=206 ttl=39 time=84535 ms
64 bytes from 8.8.8.8: icmp_seq=205 ttl=39 time=85536 ms
64 bytes from 8.8.8.8: icmp_seq=203 ttl=39 time=87536 ms
64 bytes from 8.8.8.8: icmp_seq=209 ttl=39 time=81536 ms
64 bytes from 8.8.8.8: icmp_seq=211 ttl=39 time=79537 ms
64 bytes from 8.8.8.8: icmp_seq=207 ttl=39 time=83537 ms
64 bytes from 8.8.8.8: icmp_seq=208 ttl=39 time=82537 ms
64 bytes from 8.8.8.8: icmp_seq=212 ttl=39 time=78540 ms
64 bytes from 8.8.8.8: icmp_seq=210 ttl=39 time=80540 ms
64 bytes from 8.8.8.8: icmp_seq=213 ttl=39 time=77541 ms
64 bytes from 8.8.8.8: icmp_seq=215 ttl=39 time=75542 ms
64 bytes from 8.8.8.8: icmp_seq=217 ttl=39 time=73542 ms
64 bytes from 8.8.8.8: icmp_seq=216 ttl=39 time=74545 ms
64 bytes from 8.8.8.8: icmp_seq=289 ttl=39 time=1535 ms
64 bytes from 8.8.8.8: icmp_seq=214 ttl=39 time=76546 ms
64 bytes from 8.8.8.8: icmp_seq=290 ttl=39 time=536 ms
64 bytes from 8.8.8.8: icmp_seq=218 ttl=39 time=72547 ms
64 bytes from 8.8.8.8: icmp_seq=291 ttl=39 time=121 ms
64 bytes from 8.8.8.8: icmp_seq=292 ttl=39 time=100 ms
64 bytes from 8.8.8.8: icmp_seq=293 ttl=39 time=98.9 ms
64 bytes from 8.8.8.8: icmp_seq=294 ttl=39 time=118 ms
64 bytes from 8.8.8.8: icmp_seq=295 ttl=39 time=107 ms
64 bytes from 8.8.8.8: icmp_seq=296 ttl=39 time=96.3 ms
64 bytes from 8.8.8.8: icmp_seq=297 ttl=39 time=114 ms
64 bytes from 8.8.8.8: icmp_seq=298 ttl=39 time=104 ms
64 bytes from 8.8.8.8: icmp_seq=299 ttl=39 time=113 ms
64 bytes from 8.8.8.8: icmp_seq=300 ttl=39 time=112 ms
64 bytes from 8.8.8.8: icmp_seq=301 ttl=39 time=111 ms
Hasta que vuelve a enganchar al 3.5G pues cobertura siempre tiene (-50dB/m).
Incluso a veces ni asigna IP, devuelve "0.0.0.0".
En GPRS siempre funciona.