I am a 35 year old male who can run 10k in 36mins and have done lots of half marathons/marathons. I feel very fit and run 3-4 times a week and have done for the last 3 years, but i have had a number of worrying events happen while running. Every 3 or 4 months I feel a strange sensation in my chest and feel very tired/out of breadth. I more or less have to stop as it will not clear but if I do stop then after 10 seconds it clears and I can run normally again. I thought some kind of arrythmia. I've had an echo, stress ecg etc and all normal. More recently I've occasionally had a stich like pain in the centre of my chest while running. I can run through it and other than the pain there are no other symptoms. My GP just says there is no problem but I wonder if I need more tests, eg stress echo etc. Any ideas? Or is it my heart just telling me to not train so hard? (I do push myself to max heart rate in most training runs).