Internet lags very strangely?

Hi, I have comcast internet and my cable modem is also connected to a wireless router. Lately, my ping has been jumped very quickly from 15 to 30 to 60 to 100 and then back to 15, very consistently. Does anyone know a cause of this? At first I thought someone was stealing our internet (even though it is encrypted) but I directly connected my computer to the modem without the router and still had this non-traditional lag, so I no longer think this is the problem. I have never encountered a problem like this, where my latency would fluctuate so quickly, while usually if I am "lagging" it stays at a high latency, but is consistent at that latency. Do you think it is a problem with my ISP or maybe like a virus on my host computer? Thanks.