First:
Test more than one server from more than one company and in completely different locations that aren't anywhere near each other (for instance a server from ilan in NY and a server from GR in Dallas). If the problem continues then move on to step 2, if problem ceases on some server yet exists on others, continue to step 2 but work only the ip's that you are recieving packetloss.
2: Tracert each of those places that are major locations on the internet (google.com yahoo.com msn.com etc...) and find out exactly what hop your ping spikes/gets dropped. Save the tracert's.
3: Contact your ISP and tell them that there are issues with x hops to different locations around the country, that you're paying good money for a service that's not letting you do anything. Proceed to threaten to get a new ISP, results ensue more quickly this way
4: If they're a good ISP then they'll bend over backwards to help you and fix the problem. If the bad hop is outside of their network, then they may do one of a few things: route you differently. if this is the option they provide this may completely fix the problem, but be sure to give them specific ip's you want to be able to ping well to. if they do not route you through a different path then they will contact the facility in charge in order that they can fix the bad hop.
if the bad hop is on their network then you're most likely not going to be the only person having this problem, and bringing it to their attention will help build up a complaint list so long that they can't ignore it. also threaten to change isp's.