Unstable ping times to client PC

I have a client complaining about bad connectivity.

Her PC is connected via the power shot towards a rb133c-5Ghz-antenna combination via a wireless network to the internet, as all of my other clients are.
Connectivity to her antenna is near perfect and its up for many days.
All diagnosis to her antenna are fine and each time when I visit the client nothing is wrong.

Now I set up a running arp-ping (normal ping is blocked) from the rb-antenna to her PC and I see a very irregular return time.
1ms to any figure up to timeout (1000) while there is no other traffic running.
After 10 mins or so no more returns while link is still showing “ok”
I stop the ping and start is after a minute or so again and the returns come up again, with the same variable return times…

What does this tell me?

Bad adapter of her PC?
Bad performance of her OS (Windows)
Bad performance of her hardware? (Warm cpu, low memory, old PC) and thus bad performance of her OS.
Bad connection of the utp cable between PC and powershot? (I replaced rj-45 plugs, used brand new crimp tool, replace cable by proven proper one etc. nothing helps)

It drives me nuts… why is her connection failing so regularly? And why her pings so unstable?

Any suggestions?