Unfortunately, connections are fairly complex. Any part of the network could be causing the issue, anything from your service plan, to your wireless card in your laptop, to the router, to just not being in good range of the router. Hell, it could even be that if you're between a few walls or electronic devices, that if something turns on, there would be enough interference between your computer and the router to cause intermittent problems. This wouldn't be noticed in short tests; only over a longer test that could be looked at broadly.
I'm guessing you have a decent connection to your router, though I don't think I had you run a
speed test. Your ping suits something that is some type of broadband, and I'm guessing bandwidth isn't an issue.
If you have a router with antennas that protrude from the housing, you could try something like
the Wind Surfer to direct the signal to where you are usually using your computer. This would probably help a bit if interference of any kind was a problem, and it's extremely cheap and easy to do.
I know I haven't said this before, but I've got pretty much the same specs as you on my laptop, though I'm running Windows 7. More than enough to handle RuneScape. There shouldn't be a need to do anything to Java unless you've really got a
massive amount of junk running in the background that is taking a higher priority.