Packet Drops in the Internet

Dear All

Our company has got 3 locations and Head office is linked with the
branch office through Sonicwall by establishing a VPN connection in
between them.

My head office has got sonicwall 3060 and branch office has got
Sonicwall TZ 170.

In the branch the Internet is connected via 1 MB ADSL Modem -->
Sonicwall TZ170 --> Switch.

In one of the Branch we got 1 Server which updates its database with
the Server in the Headoffice in regular intervals of time.

When we ping the Sonicwall from the Headoffice , it took 50 ms to get
the response.
But the past couple of week, it is taking almost 250ms and in the
meanwhile the packets are getting dropped.

I did the following
1.Checked with the ISP vendor about the connection, they say it is
fine by connecting from their location
2.changed the ADSL Modem
3.Changed the Sonicwall

Still we are getting the same problem.

Yesterday I found that , If i restart the server in the Branch
office , I am able to get the ping response for 50ms. But after 1/2 hr
still the same probelm (response time is more and packets drops)

Guys could you please help me know where is the problem and how to
solve this problem
NOTE: Other branch which connects to the head office is working well
( no packet drop)

Thanks in advance
SAM