Tacx vortex smart disconnect when i stop pedaling?

Hello and sorry for my bad english : i try to explain you my problem : i do mountain road with my Vortex smart T2180 : during 1 hour : everything is ok : when i climb the HT become more harder : i use an IPAD ( bluetooth connexion )
After one hour and a half : the Home trainer stop become more harder when i climb
each time it s like this :frowning_face:
do you think it s because i stop pedaling for a few second ??
can you help me plz
tacx utility is update
bests regards