Exercise induced arrhythmias

I am a 36 year old male. I have stayed I pretty good shape, I'm about 15lbs over my target weight. I eat a very good diet and workout 5 days a week, cardio 5 days 30 mins and 3 days of weightlifting. I have experienced tachycardia since the age of sixteen, very infrequent. About five years ago I started a new job and quit exercising. One day I was carring something heavy and noticed an extra beat between my normal fast rhythm, the rhythm seems the be steady and normal with no symtoms (symptoms). I mainly notice the beats during panic attacks or most during lifting heavy weights or vigious cardio, but mainly I notice these during exercising while I'm tired or sometimes when I need food. I have been to the cardiologist twice over the last three years. He has preformed two ekg's, two echo's, and one holter test for 48 hours. He says that I have PAC that is probally caused by adrenaline release. This condition makes me very anxious!!

Does this condition happen often and is it dangerous?

Can you exercise while you are having PAC's? Is that safe to keep going?

It happens often at when my heart rate goes over 160 bpm, How high should I push my heart rate?