Having gone thru like 12 flatbed scanners in three years (4 Canon 9950's, 1 Epson 3200, 2 4870's, 3 4990's, two Microtek 1800f's) and with seven failing in the first week. I have some suggestions to test yours for manufacturing defects when it comes out of the box.
1)Look for crap, dust and fily residue underneath the glass. You can clean this by disasembling it but you shouldn't have to and if you do you will invalidate your warranty.
2) do a 100% max dpi RGB scan first thing of a neg or trans. that has clear blue or gray skies. Look at the scan at 100% and check for alignment by checking whether the scan is evenly sharp on all edges. If the edges are all sharp but the center is out or vice versa that is generally a film flatness issue which can be dealt with rather than an alignment issue. If the corners are not evenly sharp return this puppy. It can't be fixed by you.
Also look for streaking in the direction of the scan. This will not get better.
Also look for stretching of the grain in the corners and ghosting. Stretching has to do with sloppy tolerence in the belts. Both of these are common in the Epsons and you can live with it if you don't print above 16x20.
3) do the same scan with Digital Ice on and check carefully for banding (banding is a repeated pattern) in the skies. This is the most common place that banding will show up. If you see banding return this puppy. It can't be fixed by you. Do this even if you never intend to use DI. This exposes a hardware flaw.
4) Repeat with DI on 2) but apply a steep curve to the prescan. This will also show more subtle banding problems that may inadvertantly creep up on you trying to scan a flat or underexposed negative. Do this even if you never intend to use DI. This exposes a hardware flaw. Return it.
5) Are the scans less sharp than you expect (your expectations might be too high if you are used to drum scans)? Put some paper shims under the film holder and scan again. Do the corners get sharper? If it does the focus is off. Return it! It is a manufacturing flaw! You can't fix this and you don't want to have to use shims every time you scan. Lay a 4x5 directy on the glass emulsion up and repeat the test. Is it sharper? Return it or keep this one for wet scanning! It is a manufacturing flaw! But you can make it work for you if you wet scan.
This testing allowed me to pick up defects in 7 scanners right out of the box and return them till I got a good one. What do you do to test a scanner?