Ok I have downloaded the manual and found the section copied here from page 74. Copying it has lost some graphics so do refer to your manual for full clarity.
To me it reads that to take a still shot while recording a movie will only happen if your choice of memory card speed is high enough and other conditions. If any or all of the conditions are not right you will not be able to take those still images.
Note ~ you mention lags/pauses on video, this to me suggests your memory card does not meet the specification needed to record video as well as possible and that based on one of the criteria copied below is a reason why you are not able to take still images
Please state exact specifications of the memory card you are using.
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Shooting Still Images during Movie Recording
Still Images Movies
Shoot still images during movie recording as follows.
1 While shooting a movie, focus on a
subject.
z Make sure that [ ] is displayed, and
then press the shutter button halfway.
z Focus and exposure are now readjusted.
(However, the camera does not beep.)
z Movie recording will continue.
2 Shoot.
z Press the shutter button all the way
down.
z Your shot will be recorded.
● Shooting still images during movie recording may not be possible
when the memory card’s recording speed is slow, or when there is
not enough free internal memory.
- [ ] (blinking): Image processing for recording is in progress.
You can shoot when the icon stops blinking.
- [ ]: Still images cannot be shot.
In [ ] mode, [ ] is displayed during movie recording, and still
images cannot be shot.
● When still images are captured during movie recording, the still
image and shutter sound are included in the movie.
● The flash does not fire.
● Movie recording may stop if shooting still images fills up the internal memory