A little late to the party, but this describes me almost exactly. Most exercises will trigger them sometimes, but squats, overhead press, and standing curls cause them reliably. They are definitely worst with overhead press where I'll get them almost every rep. They go away almost immediately after putting the weight down. And like you, cardio doesn't reliably trigger them. I only get them at rest when I'm nervous about something (like sitting in the waiting room for an EKG)

My doctor ordered an EKG and upon review it was normal and he cleared me to continue lifting and said everything was fine. Being that they're quite distressing, I just asked him to order me an ECG and a stress test which I still need to do. I'll update once I get those.

Have you had any tests done for this yet?