most likely cord problem. RMA it or replace the cord with something else (another g400 cord works but its shit so use something else mx518 or a razer cord or something, most will fit np).

if its just minor skipping it could be the lens over the sensor seated badly or dirty so open the mouse and clean+reseat the lens (you can try this first cause its easier). but g400 is notorious for cord probs soz

There are screws underneath the mouse feet.

I think I've fixed 8 Logitech mice for the cable issue now, here's what I do:

• remove the mouse feet (be careful, it's easy to separate the sticky from the pad itself)
  
• remove the screws and open the casing
  
• remove the USB cable
  
• cut out a section of the cable where it exists the mouse (around 1.5")
  
• splice the cable back together inside of the mouse and put it back in place
  
• assemble the casing and replace the feet

You could also get the g400s.

Yep, I brought a MX518 cable off ebay for about £5.
Do the the post above suggested, here is some photos of the process.
http://www.flickr.com/photos/photonic_cocktail/sets/72157632282574544/

You should buy new mouse feet as well afterwards, some can be found on amazon. If you are careful with replacing the feet you may not need to buy new ones.

You can also secure the cable internally so stress isnt put on the connection between the cable and the mouse pcb, i used a dab of non conductive epoxy to secure the cord to the mouse's shell in my friend's g400 so it would stop breaking like this.

http://logitech-en-emea.custhelp.com/app/ask

They'll send you a free g400s.

I had the same problem with my g5 well after warranty. It's actually opened up on my table with my soldering iron right now. After that broke i was using a g400 that developed a scroll wheel problem. I told logitech about it and they sent me a g400s in under a week. I didn't even have to send back the g400, and since it was a minor scroll wheel problem i can still use it if i had to. Basically i'm saying their customer support is great and you should send them an email. If that falls through do what TechDude said as long as you have access to a soldering iron.

Out of curiosity, if I RMA'd my old mx518 would they send me a g400s as replacement?

I had this problem with the g400 also, it was really annoying. So i tried, like TechDude said, to cut a piece of the cord. And now it works like new.

You have to haggle, they'll try to give you a g300 or a g500 for some reason, you have to tell them to give you g400s