Sounds like a problem with the physical hook of the phone.
Try to tap on the hook-button of the phone a little bit harder when this happens again.
I know this with some older 7940 and 7960.
When they are out of warranty, open thm and clean the mechanical part of the hook, before reassebling use a LITTLE amount of grease to put it on any moving part of the hookswitch.
I hope this helps
Greetings
Chris