sudden dive

[erhard52]

i wonder if anyone has a possible explanation for the following model behavior with nano receiver.

first time i crashed a larger glider 6 years ago the following way: i did i range test as recommended and forgot to remount the transmitter antenna. nevertheless i flew on a slope and the plane flew beautifully but then went into a sudden dive, i obviously pulled the elevator but nothing happen and just before crashing it recovered. oh i thought what was that and kept flying and it happened again, same thing it recovered. no i thought oh i better land. so i flew a larger loop behind the hill made a turn and the plane flew towards me ready for landing and suddenly when into a dive again and crashed. 
obviously i blamed the missing antenna. i am not sure how fail safe is set but when i just turn off the transmitter all servos stay where they are. 

second time just 1 week ago: another glider, flying happily and same dive behavior and plane luckily recovered. i thought it was maybe a down draft in the hills but way would it go in a real dive as if pushing down the elevator? after inspecting the plane i thought i found an issue with a lose vtail mount which allowed the vtail to slide along the fuselage. obviously nit good, so i fixed it thought that was it and following 5 flights or so where fine.
today i flew again and again the sudden dive happened and luckily i could recover just before crashing.

so what can this be?
down drafts don't seem to really explain the sudden dive
losing RC link should leave the servos in place and not cause the dive

any ideas what be great because it makes me very uncomfortable 

btw: transmitter is a multiplex EVO 16 with XPS RF module

thanks
erhard