Personally i'd check the firmware on your router is current and try and investigate the issue, but that's just me.
Personally a single box set-up is rarely the best option, I choose to split my modem and router as well as wifi, this means if I reboot the router, the line still retains sync. If I want to run a 3rd party firmware such as Tomato or Merlin I can, and if I want to extend my wifi or increase it's speed it's easily done.
My current weapons of choice:
ECI/HG612 - depending on the cabinet, don't pay a premium for an 'unlocked' HG612, it's a 2 min job.
AC56U - same CPU/RAM as AC68U but slower wifi but that's not an issue for me because....
2 x Unifi AP - seamless hand-off and great coverage.
Since then things have moved on a little, i'd like to have hardware acceleration for VPN encryption so i'd probably look at the MikroTik RB750GR3 hEX lite to replace the AC56U, it's cheap (circa £50) but the learning curve on RouterOS is a little steeper than the Asus, i'd also have a look at the AP range from MikroTik as well, but in my case wifi is for phones/tablets, anything intensive goes via cable so coverage and stability was the priority and the Unifi AP's are more than capable of saturating my WAN and have been faultless.
PfSense is a decent shout, again a steeper learning curve, but a lot more flexibility. The issue I have with an old PC is power. An old PC running for 2 years will cost you more than buying decent kit in the first place, if it's old now, then it'll be worth bugger all in 2 years. If you already run a local server anyway then going the VM route may work, all depends on what you have/want.
