Just thinking as an objective observer who doesn't know you or your general health, age, history, etc, it seems to me that your heart just goes off once in a while. It might be stress, might be illness, might be....something you ate, thought about, feared....whatever. But the point is that you aren't in tachycardia most days and for an hour or more at a time. You have this seldom. So, with a prescription for diltiazem in hand, and IF you know for a fact that it does work for you when you take it (when you need it), I don't see why you should be unduly anxious or let your travel plans go by. What you may wish to do is to keep a record of your episodes, when, how long, how they made you feel. Then, over time, you will see if they begin to come on more and more frequently, or if they (also) last longer, or make you feel more faint, dizzy, short of breath, more anxious. At some point you'll know you need to get a more aggressive treatment. You don't want the condition to get worse if you can help it, or to become more advanced and less difficult to manage, say with just diltiazem. If you keep records, you'll have something to show your cardiologist which will do two things: show him/her that you are in this all the way and serious about record-keeping and monitoring its progression.; and it will allow that person to see any changes as evidence that you have collected which may induce a prescription for more advanced care.
You are in the early stages, apparently, and this will be dangerous only in the event that it suddenly converts to a long-lasting and dangerous arrhythmia at high rate. This is not your history so far.
The only caution for you is a big one; insurers will want you to answer truthfully about all known conditions prior to embarkation. If you fib, they'll find out from a physician who has been caring for you when you go to claim for travel, lodgings, and medical treatment when traveling and things go wrong. And if you do answer honestly about an existing diagnosis of paroxysmal tachycardia, they will state up front that you are NOT covered for that while you travel under their policy. You're on your own. Something to think about.