Hey!
Tachycardia literally means "fast heart rate". In the medical community, tachycardia is classed as a heart rate of over 100 beats per minute.
Sinus tachycardia means that the heart rhythm is regular (same amount of time between beats) and the atria and ventricles of the heart are both beating at 100 beats per minute. There are different types of tachycardia and they're named based on where the impulse generates from (sinus node, AV node, or other pathway) and the chamber(s) of the heart beating over 100 beats per minute.
I have issues with my heart rate not going under 100 occasionally. It just happens for me, no rhyme or reason most of the time, and my doctor (electrophysiologist) is aware of it. If I'm symptomatic, I just sit down and try and think about what I might need (salt, hydration, rest, food, legs up) and meet that need. If I'm symptomatic with a heart rate over 100, it usually means I've stretched myself too thin and I need to rest up. Not the greatest solution when you're at work!
As for your tachy/brady fluctuations, I'd recommend talking to your doctor about that.