Reduced speed in the evenings and at weekends isn't uncommon whichever ISP you're with.
It's all to do with contention ratios (the number of people trying to access the same BB 'pipe' from your local exchange) and, to a lesser extent, your distance from that exchange.
Having said that, a drop to 200Mb from 1700/1900Mb does seem rather on the high side.
Which BB speed tester are you using to arrive at your figures?
If you can't get any sense out of AOL switch to a proper ISP such as Tiscali.