I've seen some transfer rate tests like that, with similar results. Do you also hear your drive slow down when those dips appear? Also, do you run the transfer rate test basically running by itself, or are you multitasking when this test runs? I find if I multitask when the transfer rate test runs, I'll get dips, but my drive won't actually slow down. If I let the test run by itself, the curve is normally smooth. So the biggest signal to look for is when you see a dip, does your drive actually slow down from a higher speed b/c it has trouble reading the DVD? If it does, then there's a read problem.
Others may be able to elaborate more as to how many times you might get graphs like that with certain mids, but that isn't information I can speak intelligently on.