Postgers stops accepting local connections ;

When I log into postgres host as postgres su and try to connect to DB
(i.e. psql)
I get the infamous error message:
"
Unable to connect to PostgresSQL server: connectDB() -- connect()
failed: Connection refused
Is the postmaster running at 'localhost' and accepting connections on
Unix socket '5432'?
"

The strange thing is :
-It DID work little time ago and nothing has been done to it.
-(windows) Clients CAN still connect to the DB without any problems !!
???
-I am running it logged to linux as "postgres" su.
-the postmaster is running.
-the postmaster was started "su - postgres ..." and the port is 5432
- If I restart postgres then it will work for a while, but after a while
it stops working, without any apparent reason.

It has done the same thing few times now.

Any suggestions ?
I'm new to both linux & postgres so any help is greatly appreciated.

-michael (A)