PROBLEM Service Alert: hostname/check_postgres_old_transaction is CRITICAL **

From the past few days i am getting this old transation alert from some of my DB slaves. I am using EDB9.2.7.18 on on CentOS6.5 ( Final ).  From my app boxes i got some of the transactions remains stuck for 2-3 hours which took ~ < 60 seconds in database. Below are the details :-

-- DB execution time

explain analyze select * from appusage_func('2014-09-19 15:26:15.662');

-[ RECORD 1 ]--------------------------------------------------------------------------------------------------------------------------------------
QUERY PLAN | Function Scan on appusage_func  (cost=0.25..10.25 rows=1000 width=12) (actual time=34060.657..34813.542 rows=7362310 loops=1)
-[ RECORD 2 ]--------------------------------------------------------------------------------------------------------------------------------------
QUERY PLAN | Total runtime: 35174.296 ms

It returns approx 311 MB data to the client servers.

root > netstat -p | grep 45355
tcp        0 1531648 localhost:5499              localhost:48848             ESTABLISHED 45355/postgres

root > strace -p 45355

read(306, "\30\0kB\3\0\0\0\0\0\377\5\0\0\26\0\0\0\2\0\0\0\30\0+\264\3\0\0\0\0\0"..., 8192) = 8192
sendto(10, "4174\0\0\0\0041535D\0\0\0\30\0\2\0\0\0\006248710\0\0\0"..., 8192, 0, NULL, 0

Then after 2-3 hours it got completed automatically with below stacktrace :

# strace -p 45355
Process 45355 attached - interrupt to quit
sendto(10, "4174\0\0\0\0041535D\0\0\0\30\0\2\0\0\0\006248710\0\0\0"..., 8192, 0, NULL, 0

        ) = -1 ECONNRESET (Connection reset by peer)
open("/usr/share/locale/en_US.UTF-8/LC_MESSAGES/libc.mo", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/usr/share/locale/en_US.utf8/LC_MESSAGES/libc.mo", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/usr/share/locale/en_US/LC_MESSAGES/libc.mo", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/usr/share/locale/en.UTF-8/LC_MESSAGES/libc.mo", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/usr/share/locale/en.utf8/LC_MESSAGES/libc.mo", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/usr/share/locale/en/LC_MESSAGES/libc.mo", O_RDONLY) = -1 ENOENT (No such file or directory)
write(2, "\0\0J\1+\261\0\0t\35\230\2\1\0\0\0002014-09-19 18:04"..., 346) = 346
write(2, "\0\0.\1+\261\0\0t18:\1\0\0\0002014-09-19 18:04"..., 318) = 318
close(306)                              = 0
stat("base/pgsql_tmp/pgsql_tmp45355.36", {st_mode=S_IFREG|0600, st_size=161970820, ...}) = 0
unlink("base/pgsql_tmp/pgsql_tmp45355.36") = 0
sendto(9, "\17\0\0\0\30\0\0\0\230\275\22\0\0\0\0\0\204z\247\t\0\0\0\0", 24, 0, NULL, 0) = 24
write(2, "\0\0\256\0+\261\0\0tFf\0\1\0\0\0002014-09-19 18:04"..., 190) = 190
close(305)                              = 0
close(264)                              = 0
sendto(9, "\2\0\0\0\250\3\0\0\230\275\22\0\10\0\0\0\1\0\0\0\1\0\0\0b\366\7\0\0\0\0\0"..., 936, 0, NULL, 0) = 936
sendto(9, "\2\0\0\0\250\3\0\0\230\275\22\0\10\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 936, 0, NULL, 0) = 936
sendto(9, "\2\0\0\0\250\3\0\0\230\275\22\0\10\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 936, 0, NULL, 0) = 936
sendto(9, "\2\0\0\0\250\3\0\0\230\275\22\0\10\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 936, 0, NULL, 0) = 936
sendto(9, "\2\0\0\0x\1\0\0\230\275\22\0\3\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 376, 0, NULL, 0) = 376
sendto(9, "\f\0\0\0000\0\0\0\230\275\22\0\1\0\0\0e\346_\0002047\1\0\0\0\0\0\0\0"..., 48, 0, NULL, 0) = 48
write(2, "\0\0\325\0+\261\0\0t\35\230\2\1\0\0\0002014-09-19 18:04"..., 229) = 229
exit_group(1)                           = ?
Process 45355 detached

I am sure there will be network issue but what might be the reason for that. It comes 2-3 times daily.

Please let me know if any other information is required.

Thanks