High Ping

My internet speeds have drastically dropped lately, and after many talks with COX, they sent a tech to my house.  He couldn't figure out the problem either, and ended up replacing my modem which worked for the low speeds, but now my ping is incredibly inconsistent.  When I do ping tests on my computer, sometimes I'll get under 60 ms, and sometimes I'll get up in the 100's.  What could be causing this high ping?