I will assume that it's a NEW 7900, which means you have a warranty. Therefore the first thing to do is call Epson. Check the book(Epson Preferred) that came with the printer and on page 4 you'll find all of the info you need to get in touch with Epson Tech Support. Since you are a new poster you may not have read any of my previous posts concerning my 9900. If not I'll just say that I consider myself to be somewhat of an expert on dealing with Epson Tech Support, and leave it at that.
One other thing you might want to consider. That is of course extending the warranty beyond the one year period, and you can do that anytime before the initial warranty expires. Either one year at a time or two years. All of that info is on page 3. It was the first time I have ever bought an extended warranty for anything, but as the end of the first year was drawing near I felt that I had no choice. I consider it to be the best insurance policy one can buy for these printers. If in doubt read this thread - http://www.luminous-landscape.com/forum/index.php?topic=61585.0
. It's a long thread but very informative. My last warranty is nearing the end and what I have learned from Eric will I'm sure be put into good use at some point. The "wiper" replacement info has already been very useful. By no means do you have to read the whole thread to get the point, but a glance at least might convince you of the value of the warranty. My 9900 has been paid for twice with the extended warranty, according to my calculations and info from the techs who have worked on it.
I apologize for taking you down this loooooong road, but food for future thought perhaps. Call Epson and get them working on your issue. They'll start a log and any further problems will be added to that log. In fact I created an "issue log" when I first started up the 9900, it now has 101 entries.