It sounds like you may have ANS which is very common with some of the symptoms you're experiencing; sometimes even autoimmune issues are cross linked, but you need to find a specialist to make sure. How is your blood pressure?
Sometimes there is a simple miscommunication between the heart and the brain. If you stand up, move or exercise - this takes more blood away from the central part of the circulation where it is needed, as a result we may feel lightheaded, dizzy or faint because not enough blood is getting to the brain.
Dehydration from things like alcohol, exercise or being in the heat may make symptoms worse, so try to avoid these triggers you have until you get answers. You could ask them to do a tilt table test to see what happens to your blood pressure and heart rate.
Pvc's in a normally structural heart are 95-99% benign, and they would most likely see any cardiac problems in some of the testing you've had done. Have you seen an Electrophysiologist not just a cardiologist? You need to find out exactly what it is that you have and what your triggers for that type of arrhythmia is, not everyone is the same.
You should see an ENT (ear nose throat) also just to rule out anything such as Grendslori says. I don't think this was caused by infection which is a common cause of dizziness because you had blood work done, which they would have seen.
I have major ANS (autonomic nervous system) cardiac related issues and I've had lightheadedness, dizziness and fainting and arrhythmia's such as PVC's for over 35 years. I was told from the age of 9 that I did it for attention, then anxiety, then stress and finally at the age of 42 a doctor took me seriously and I found out my problems are cardiac related, which is very rare.
Anxiety can worsen your symptoms because too much oxygen reaches the brain, so try to control it when you start feeling those twinges of higher heart rates - breathe in through your nose for 10 seconds slowly, then breath out through your mouth for 10 seconds slowly see if that helps until the HR slows down. Also, a cold glass of water will help very quickly calm you, plus help lower higher heart rates and help if you're dehydrated.
ANS issues are very difficult for doctors and patients to understand, most of us go many many years being mis diagnosed, until someone gets it right.
Here's some info on ANS issues -
http://www.medhelp.org/health_pages/Neurological-Disorders/Autonomic-Dysfunction-FAQ/show/181?cid=196 look at the Uncategorized section - you have some symptoms in there but may be completely unrelated.
Here's a list of doctors to see if any are around your part of the world:
http://www.medhelp.org/health_pages/Neurological-Disorders/Dysautonomia-Specialists/show/717?cid=196
Here's some more reading and info:
http://www.medhelp.org/health_pages/Neurological-Disorders/Autonomic-Dysfunction--Dysautonomia-Index-Page/show/857?cid=196
just throwing some thoughts out there, hope you feel better soon and find some answers.