Hmm... looks like hardware isn't a problem, since 2 years ago
the hardware is able to support higher speeds.
I'm wondering if using Starhub and Singtel would actually split the
connections (as in causing the speeds to half, since 2 PCs are
connected to the Net right now).
Starhub doesn't have a dedicated bandwith, so if at any one time
more than PC is connected to the Net, the bandwith would be
lowered, causing the speed to lower as well.
In this case, surfing speeds of 3k to 5k is normal.
Your second question - notebook uses both wired and wireless
connections. Wireless when you plan to move around and connect to
the wireless hotspots around Singapore. Wired is when you are at
home or if you have no wireless products.
Third question - Not really sure, could be due to having 2 ISPs
again. Try disconnecting from one of the ISP and see what
happens.