Well, to start with, I (and a lot of others) know exactly how you feel! The somewhat good news is that you probably aren't going to die from the PVCs if you've been checked out and ruled "benign". I just completed a 9 month or so streak where I was getting 10k+ per day. I'm on Metoprolol 25mg which perhaps is similar to your dosage of Atenolol. I have the same side effects as you experience with the dizziness (esp. when standing) etc.
It's really hard to find what causes these things. Even if you find something that you think causes them, it could have been something that changed a week or 2 ago or some other hormonal or chemistry change.
I'm in remission now for a month or so. Was it because of less stress at work? Maybe. Was it because I started 2g of Taurine and 1g of L-arginine a week or so before they stopped? Maybe. For now, I'm happy they are gone for a bit!
I'd suggest you don't give up on trying to find the cause. It sure does seem like in a lot of cases the cause is different from person to person. Magnesium didn't do anything for me either. If you haven't tried it yet, give Taurine/L-arginine supplements a try for a week or 2 and see if it does anything for you.
I know it is hard to do but the other thing to do is try to ignore them. Trust me I know how difficult that is!