Mass effect 3 multiplayer

I usually find matches in less than 2 seconds, but sometimes it searches infinitely and the only thing I can do is restart.

Have you tried hosting a game? See if people are able to join you.

I just tried yesterday eve, no problems finding others for Silver (PC, US)

How are you trying to join games.  "Quick Match"?  Or "Create New Match"?  Both?

You say you have all the DLC, but might be worth checking anyway ...

An easy way is from MP character select screen, all characters should be visible (even if still locked). For example, there should be 11 Adepts shown.

Or (but I recommend above):

There should be five (for MP): Resurgence, Rebellion, Earth, Retaliation, Reckoning

They are located in your install folder.  The last few sub-folders are:

Origin Games\Mass Effect 3\BIOGame\DLC

There should be 5 files named: "DLC_CON_MP#", where # = 1 through 5