Dymag problems, any ideas?

Did my first track day this year couple a days ago, went fine in the morning, but in the afternoon the bike starts sliding, so came in checked rear tyre and found it was flat! Couldn't find any damage but found the metal valve stem lock nuts slightly loose (these are dymag 5-spoke magnesium rims, second hand but immaculate condition and nearly new).
Went out for the next session and 15 mins later same problem, checked tyre/valve with water for leaks and was ok, pumped up and left overnight, still ok not lost any pressure.
The only things I can think off is it is a puncture but is only apparant under load, or the tyre is moving on the rim (doubt it though), or the valve cap is the annodised type but has no seal inside (neither does the front and thats fine), I have heard that these type of caps without a seal can release the valve and let the air out?
Anyone got any thoughts on this, I need to get it sorted before my next outing in 2 weeks.

Help please