Bicycle Tutor

Hello, having a bit of a problem with shifting. My bike is only about 4 weeks old and I do about 10-15 miles daily on it. I can shift through all gears (21 speed) with no problem, but after shifting into 3rd on the left and past 4th on the right, the chain will slip and jump all the way out to 7th gear. If I shift it back down to 4th gear or lower, or move the grip shifter into 7th, it runs smoothly and accurately again, but if I try to leave it in that 4th-6th gear range the chain just slips and jumps back out.
Any ideas?

This rear derailleur video may help. If you have adjusted it and it still causes you trouble the derailleur hanger may be bent (usually happens if the bike falls over onto the derailleur).

Here's a video that shows how to align bent derailleur hangers:
http://bicycletutor.com/derailleur-hanger-alignment/

This can be a problem with some cheaper department store bikes. It can even be difficult for an experienced tech to adjust out.