Kinda same problem, somedays my ping is around 55 as usual but suddenly raises to 80-120 w/o anything changing in my comp, and i live in about same area as you, u know what is really odd? Well my friend and brother have been living in my place and we use the same router and internet, but they dont have any problems with ping...
Btw we have Sonera 100/40 or something like that, what you using?