I assume you're running both Tvheadend server and client on the same Pi.
I had the same issue with my DVBSky s960 + RPi3. Here's how you can fix it:
1) Switch from Advanced deinterlacing to Bob. Advanced deinterlace is using a lot of RAM bandwidth and it may cause issues (especially on HD channels) when both PVR client and server are on the same Pi. This is not the best solution because Advanced looks better than Bob.
2) Increase the "arm_uc" value (I use 12 0). The default value for Pi3 is "10 0". For Pi2 it is probably the same. Run "vcgencmd arbiter status" to check the current value.
To make the new setting permanent add this to autostart.sh file:
^This setting completely fixed the issue for me, I didn't have to disable the Advanced deinterlacing.
RAM overclocking will also help a bit.
Also, see this thread - thread-4235.html
Linux kernel 4.9.x perform poorly on the Pi. The latest LibreELEC builds are based on kernel 4.9.x. Use a build based on pre-4.9 kernel.