I came across a little mention on this issue and it stuck in my head as I was having a similar issue many years ago. Basically the DA cranks that came on my first track bike about 11yrs ago never loosened. Then I decided to go to 175 cranks and got new DA cranks and they were constantly coming loose such that I had to tighten about every 2 weeks or about 6 training and racing sessions. I came across a snippet from someone that said that newer bolts were ever so slightly different in dimension to older ones. So I still had the old cranks and bolts so I re-used the old bolts and never had a problem since. Until going to Vision cranks in November I was still using those original bolts and zero issues. Now I don't know just how correct that info I found was, but when I went back to the older bolts and zero issues it was enough for me to say there's definitely something there. A dab of blue locktite might do the fix for you