There are many causes of palpitations. Blood pressure itself is not considered one of them (I could be mistaken though). Anxiety, caffeine intake, alcohol, smoking...are frequent causes of palpitations. Have you been told yours are from blood pressure? Most of the time, palpitations are harmless. It is when they are caused by arrhythmias that there can be reasons for concern. However, certain drugs that reduce resistance in circulation can be responsible...guess what those drugs are? Nitrates and calcium channel blockers.
As for the stress test, it is NORMAL and expected for blood pressure to rise dramatically, as long as it remains within bounds. The test may be stopped if the blood pressure rises or falls beyond acceptable levels. It is normal for systolic blood pressure to rise to 200mmHg or more at peak exercise. Diastolic usually remains unchanged or falls only a little bit. If the person being tested has hypertension, there will be a rise in both systolic and diastolic pressures. Diastolic pressure can rise to 100 mmHg or more.
If significant ECG changes are present at rest (before the test is done) - such as in people with long-standing hypertension, pacemakers etc. -reliability of the test is drastically decreased. Its reliability is quite limited anyway (as you probably know). The echocardiogram (did you have a resting echo or an exercise echo?) would have shown any structural & functional irregularities. By now at least two doctors have reviewed the results.
You are entitled to seek a second opinion, certainly, if you feel you should. You could also talk to your doctor and ask how high your blood pressure climbed during the test (and whether your blood pressure was the reason for stopping the test).
Good luck -try not to worry too much!