Sometimes when you are not getting straight stitches (they appear to be at a bit if a slant) is an indication that your bobbin thread is too loose. But that wouldn't be the reason why you are getting skipped stitches.
Are you sure you have your needle in correctly? The flat side of the needle goes to the back of the machine. Also, is the needle all the way up against the stop? When I put in a new needle I hold the needle cartridge under the needle point pushing up so I'm sure to get the needle in properly.
Also, do you have the settings on your machine set correctly for a straight stitch?
Good luck and I hope this helps. If you do take it back in for repair, please post what the problem was, we are all curious.