That sounds exactly like what I have. I've had the "pause--THUD" for 10 year and finally mentioned it to my doctor, who hooked me up to one of those 24-hour holter monitors, and it turned out I was experiencing harmless premature atrial contractions (PACs). That's all, no medications or follow-ups needed. Apparently PACs and PVCs (premature ventricular contractions) are very common and are not considered a threat to heart health, even if you experience them frequently.
They can FEEL scary though, so you might want to try a few things to avoid them. To start, cut out all sources of caffeine from your diet, including coffee, tea, and chocolate. Make sure you're getting plenty of rest and that you're sticking to a regular sleep schedule. Tiredness can bring on stronger PACs/PVCs. Are you female? If so, be aware that hormone changes can trigger PACs/PVCs as well. They might worsen during pregnancy or around the time of your period.
Personally I've found that the best cure is sleep... Even a quick nap can "reset" my heartbeat so I wake feeling normal. But you might find something that works better for you.
If these irregular beats still bother you, go back to the cardiologist and tell them your concerns. They should be able to prescribe medication to "normalize" your heartbeat, and hopefully give you peace of mind. This medication was offered to me but I declined because my PACs don't scare me anymore. They're just something I learn to live with and ignore. =P