Focus problem with nikon d3100?

I have just had the camera fixed because it had a lot of problems focusing, so they replaced part of the front body unit. However I'm still having some problems. It seems to work fine using the viewfinder, but when I use it in live view sometimes it will not focus on anything for 5 minutes at a time. It tries to focus but it won't focus on anything, then if I wait a few minutes it will be fine to focus on the exact same things. Not sure if it is becaus I'm in a room where the lights are on but they are dull, or because it is a cheaper camera or because there's something wrong?

It's not the greatest to focus in video. Try outside, see if that makes a difference

If your goal is to shoot video with a dSLR, you need this. A focusing aid and then manually focus your shots

http://www.switched.com/gallery/focus-aids#/2