Server stops responding randomly for 5 minutes

Sometimes per week server stops randomly responding for approx 5 minutes.
User should wait for 5 minutes before server responds.
Other users can work normally at same time.

Monday this happens at 12:16
I havent noticed anythis special in PostgreSql and windows logs at this
time.

How to fix or find reason for this ?

Andrus.

Config:

Win 2008 server 2 GB RAM
PostgreSQL 9.0.4, compiled by Visual C++ build 1500, 64-bit
10-25 users manually entering sales order, e.q. low server usage

Server is installed using setup program from postgresql.org directed site,
postresql.conf file is not modified after installation.

Database size: 2165 MB

Biggest files in decreasing order of size:

1 firma1.bilkaib 880 MB table
2 public.attachme 454 MB table
3 firma1.rid 401 MB table
4 firma1.omrid 88 MB table
5 firma1.omdok 79 MB table
6 firma1.bilkaib_cr_idx 75 MB index
7 firma1.bilkaib_cr_pattern_idx 74 MB index
8 firma1.bilkaib_db_pattern_idx 74 MB index
9 firma1.bilkaib_db_idx 74 MB index
10 firma1.klient 69 MB table
11 firma1.dok 64 MB table
12 firma1.bilkaib_kuupaev_idx 60 MB index
13 firma1.bilkaib_dokumnr_idx 53 MB index
14 firma1.bilkaib_pkey 43 MB index
15 firma1.rid_rtellimus_idx 38 MB index
16 firma1.rid_toode_idx 25 MB index
17 firma1.rid_toode_pattern_idx 24 MB index
18 public.strings 23 MB table
19 firma1.rid_inpdokumnr_idx 19 MB index
20 firma1.toode 17 MB table
21 firma1.rid_dokumnr_idx 16 MB index
22 firma1.rid_pkey 16 MB index
23 firma1.summav 15 MB table
24 public.report 13 MB table
25 public.desktop 9784 kB table
26 public.mailbox 7128 kB table
27 public.localfil 5584 kB table
28 pg_toast.pg_toast_36145_index 5392 kB index
...