It's not lack of skill, it's normal for the heart rate to do that. The reason I never got a dopplar was because I knew it would just give me one more thing to worry about if things weren't perfect everytime I listened.
I experienced a bout of preterm labor at 26 weeks and since then have had weekly checkups where the baby's heart and my contractions were monitored for 1/2 hour. He normally stays in the 140-150 range, but has been up to 165 sustained for a few minutes and down to 135 or so sustained for a few minutes. Every once in a while he'll drop down to 110 for a few seconds--that's freaky, but the doctor said it's perfectly normal until late pregnancy (34-36 weeks or so). The central nervous system controls the heartbeat and is not completely mature until that time. He said normally a woman wouldn't be hooked up to a fetal monitor until at least 34-35 weeks and that's why you never hear about it. You can ask your doctor for reasurrance.