Perfomance problems

Hi

I'm building an app (A) that inserts about 30.000 - 40.000 records in a
table per day. Another app (B) get this records and process it, an then
the records are deleted. The A app gets 50 records each time, this SQL
statement takes 10-15 seconds. I make a vaccum and the time is now about
5-10 seconds, bu it's to slow. It enqueue records, about 10-20 in each.
The B app takes less than a second in process each message.
The statement is
SELECT * FROM table_a
WHERE number = 123
AND msg_orig = 'A'
ORDER by id;

The table is:

id serial primary key,
data varchar(260),
number varchar(12)
time_a timestamp,
time_b timestamp,
msg_orig varchar(3)

Indexes:
Number,msg_orig, id, time_b

Please, Help, What I'm doing bad?
I'm using 7.1.2 in Solaris.

Thanks.