This may sound completely absurd to you but,check the condition of your motor mounts.seriously my dad repairs them and even does warranty work.If your mounts are shot or,loose this can and will cause it pop out of gear.Also check the tranny to engine bolts because some of them can go long enough to loosen these as well.I've seen many of them that backed out far enough that,I could remove them by hand with no tools.But if you do lots of high speed driving you're proably looking at synchronizers,input gear retainer,or simply dried out shift lever seal.Good luck!