[NEED HELP] Varnish stops waiting backend after 1 second

[Garau Simone]

this problem is gonna make me crazy: my varnish istance stops waiting for
a backend response after exactly 1 second.
Every first call to a page is a 503 Backend

Daemon is configured this way:
DAEMON_OPTS="-a :80 \
         -T localhost:6082 \
         -f /etc/varnish/default.vcl \
         -S /etc/varnish/secret \
         -p thread_pool_add_delay=2 \
         -p thread_pools=4 \
         -p thread_pool_min=200 \
         -p thread_pool_max=4000 \
         -p timeout_linger=50 \
         -p connect_timeout=300 \
         -p first_byte_timeout=300 \
         -p between_bytes_timeout=300 \
         -p send_timeout=900 \
         -s malloc,3G²

and the VCL backend:
backend default {           # Define one backend
.host = "127.0.0.1";        # IP or Hostname of backend
.port = "8080";           # Port Apache or whatever is listening
.probe = {
   .url = "/";
   .timeout = 1s;
   .interval = 1s;
   .window = 10;
   .threshold = 8;
  }
  .first_byte_timeout     = 60s;   # How long to wait before we receive a
first byte from our backend?
  .connect_timeout        = 60s;     # How long to wait for a backend
connection?
  .between_bytes_timeout  = 60s;     # How long to wait between bytes
received from our backend?
}


Here is the one call in the log:
*   << Request  >> 3440734
-   Begin          req 3440733 rxreq
-   Timestamp      Start: 1462781837.623325 0.000000 0.000000
-   Timestamp      Req: 1462781837.623325 0.000000 0.000000
-   ReqStart       10.20.129.118 58572
-   ReqMethod      GET
-   ReqURL         xxxxx.html
-   ReqProtocol    HTTP/1.1
-   ReqHeader      Accept: image/jpeg, application/x-ms-application,
image/gif, application/xaml+xml, image/pjpeg, application/x-ms-xbap,
application/vnd.ms-excel, application/vnd.ms-powerpoint,
application/msword, image/pjpeg, application/x-shockwave-flash, */*
-   ReqHeader      Referer: http://xxxxxx.html
-   ReqHeader      Accept-Language: it-IT
-   ReqHeader      User-Agent: Mozilla/4.0 (compatible; MSIE 7.0; Windows
NT 6.1; Trident/7.0; SLCC2; .NET CLR 2.0.50727; .NET CLR 3.5.30729; .NET
CLR 3.0.30729; Media Center PC 6.0; .NET4.0C; .NET4.0E)
-   ReqHeader      Accept-Encoding: gzip, deflate
-   ReqHeader      Host: xxxxxx
-   ReqHeader      DNT: 1
-   ReqHeader      Connection: Keep-Alive
-   ReqHeader      Cookie: fc_uid=p;
__utma=127650066.830977012.1423064118.1426582505.1426588086.20;
_ga=GA1.3.830977012.1423064118; _gat_UA-13041322-1=1;
ZNPCQ003-38303300=71a0f671;
ff607e18ab6c715f4bb35b5bbcbe1c56=d82989olp2v0ur3gpl2ouprko6;
_ga=GA1.2.830977012.142306411
-   ReqHeader      X-Forwarded-For: 10.20.129.118
-   VCL_call       RECV
-   ReqUnset       Host: xxx
-   ReqHeader      Host: xxx
-   ReqURL         /xxxx.html
-   ReqUnset       Cookie: fc_uid=p;
__utma=127650066.830977012.1423064118.1426582505.1426588086.20;
_ga=GA1.3.830977012.1423064118; _gat_UA-13041322-1=1;
ZNPCQ003-38303300=71a0f671;
ff607e18ab6c715f4bb35b5bbcbe1c56=d82989olp2v0ur3gpl2ouprko6;
_ga=GA1.2.830977012.142306411
-   ReqHeader      Cookie: fc_uid=p;
__utma=127650066.830977012.1423064118.1426582505.1426588086.20;
_ga=GA1.3.830977012.1423064118; _gat_UA-13041322-1=1;
ZNPCQ003-38303300=71a0f671;
ff607e18ab6c715f4bb35b5bbcbe1c56=d82989olp2v0ur3gpl2ouprko6;
_ga=GA1.2.830977012.142306411
-   ReqUnset       Cookie: fc_uid=p;
__utma=127650066.830977012.1423064118.1426582505.1426588086.20;
_ga=GA1.3.830977012.1423064118; _gat_UA-13041322-1=1;
ZNPCQ003-38303300=71a0f671;
ff607e18ab6c715f4bb35b5bbcbe1c56=d82989olp2v0ur3gpl2ouprko6;
_ga=GA1.2.830977012.142306411
-   ReqHeader      Cookie: fc_uid=p;
__utma=127650066.830977012.1423064118.1426582505.1426588086.20;
_ga=GA1.3.830977012.1423064118; _gat_UA-13041322-1=1;
ZNPCQ003-38303300=71a0f671;
ff607e18ab6c715f4bb35b5bbcbe1c56=d82989olp2v0ur3gpl2ouprko6;
_ga=GA1.2.830977012.142306411
-   ReqHeader      Surrogate-Capability: key=ESI/1.0
-   VCL_return     hash
-   ReqUnset       Accept-Encoding: gzip, deflate
-   ReqHeader      Accept-Encoding: gzip
-   VCL_call       HASH
-   VCL_return     lookup
-   VCL_call       MISS
-   VCL_return     fetch
-   Link           bereq 3440735 fetch
-   Timestamp      Fetch: 1462781838.492085 0.868760 0.868760
-   Timestamp      Process: 1462781838.492101 0.868776 0.000016
-   RespHeader     Date: Mon, 09 May 2016 08:17:18 GMT
-   RespHeader     Server: Varnish
-   RespHeader     X-Varnish: 3440734
-   RespProtocol   HTTP/1.1
-   RespStatus     503
-   RespReason     Service Unavailable
-   RespReason     Service Unavailable
-   VCL_call       SYNTH
-   VCL_return     deliver
-   RespHeader     Content-Length: 0
-   Storage        malloc Transient
-   Debug          "RES_MODE 2"
-   RespHeader     Connection: keep-alive
-   Timestamp      Resp: 1462781838.492145 0.868820 0.000044
-   ReqAcct        985 0 985 153 0 153
-   End

Any precious help? Suggestion?