Undiagnosed / Unexplained Chest Pain For 10 Years

After struggling with docs for years over this I thought I would turn to the wider world some input. I am a 27 year old, healthy (enough) male who has been getting intermittent, irregular and infrequent chest pains on and off for around 10 years. 

During an 'attack' the pain can be quite severe and sharp - I guess a bit like being stabbed. The pain is localised to the left side of my sternum and feels fairly deep (i.e. it cannot be palpated). 

Attacks can last anywhere from a few seconds to a day or so, and tend to come in 'clusters' that can be present for up to a week. There can be months between each cluster of attacks (if you get my drift!).

Pain can sometimes be exacerbated when leaning forward, coughing, lying on my side or raising an arm (seems to always vary - and can sometimes not have any stimulators).

During a 'cluster' period and between attacks I can often feel a mild, underlying ache in my chest. This is easy enough to ignore however.

I have had ECGs and chest x rays that have all been fine, and am not affected at all during exercise - I can happily go out and run 10k with ease.

Doctors have in the past ruled out gastro reflux as there was no tenderness on palpation of my stomach area, although I have sometimes thought that attack can follow a big meal and be accompanied by burping (although this may be my mind tricking me). 

Musculoskeletal problems have been mentioned, but why would attacks come in clusters and seemingly out of the blue?

I am at a complete loss and doctors have been no help. My wife is scared that I will just drop dead from a heart attack - and I am struggling to find new ways of reassuring her.