Hire your own electrician to test the line, sockets and more. Usually lots of rain water creates the intermittent issues on bad/old lines as you describe. If you're looking to get a whole new line from the street then dial
1100 before you start digging but it's the best "solve all" approach IMO. Also you save a small fortune digging trenches yourself. Any sparky is able to check your wiring but ask when you enquire with them as some don't want that sort of work.
I've had good success with trades on the
Service Central marketplace, just check out the reviews first and it makes it easy to get a couple of quotes too.
EDIT: One thing to remember is since Netflix hit Australia major ISP traffic has been slowing to a crawl e.g. iiNet with free Netflix traffix. They've been scrambling to enable more equipment/throughput so you may choose to wait it out as well.