Seems like a lot of people are grousing about the MP,but Instant Action in particular.My suggestion is to play the other modes,which appear to be a little more stable,and usually yield you more XP.
My personal favorite so far is Crash Site.Good amount of XP per match,good length matches,and better spawning system where less people spawn behind you to immediately shoot you in the back.
Capture The Relay is pretty good to,especially on the lighthouse map.
So if you are frustrated,stop playing instant action and try the other modes.
I like just straight up Team Instant Action. I don't like the spawns in Instant Action, Crash Site is okay, and the other game modes don't have enough people to play with.