spikes on the ping monitor is the VM network your connected to. basically usage thats going on.
the circuit may well be crap and causing problems for your powerline connection. you say generally very stable in your other home? i did a bit of reading on the powerline kit youve got and well i wouldnt buy it. id go for TP Link products (posting from my phone)..
Powerline
200Mb >
http://www.amazon.co.uk/gp/aw/d/B004...sr=8-1&pi=SL75
500Mb gigabit >
http://www.amazon.co.uk/gp/aw/d/B005...sr=8-7&pi=SL75
Switch (gigabit)
5 port >
http://www.amazon.co.uk/gp/aw/d/B000...sr=8-6&pi=SL75
8 port >
http://www.amazon.co.uk/gp/aw/d/B003...sr=8-2&pi=SL75
there are other TP Link products. have a look
Powerline >
www.tp-link.com/en/products/?categoryid=206
Switch >
www.tp-link.com/en/products/?categoryid=204