How do you measure it?

An ECG would pick up on abnormalities in your heart rate, so maybe your own measurements are flawed.

Smoking, caffeine (somewhat), running, sitting, standing all have an influence.

All meds would have an influence, some more so, some have less of an influence.

Arrhythmias are quite common after taking meds. So that might be what causes transient tachycardia. If it's severe, a mobile ECG may be used to see whether it requires you to take meds for it or whether to discontinue a med you are currently taking.

You might want to see whether your breathing is healthy: whether you get enough oxygen through breathing. If that's the problem, focusing on it might make it worse, which is a good thing as far as diagnostics go. You can also use a saturation meter.

Do you (sometimes) have panic attacks or do you feel like you might faint?