It could be a case of having too much memory dedicated to hexxit/technic launcher, or not enough.
I was getting the "Minecraft has run out of memory" screen every 20-40 minutes, on single player.
So I decided to change the amount of memory from 1gb to 768mb, I no longer got the "Minecraft has run out of memory" screen, however I eventually did get little lag spikes that lasted about a second, but happened every 2 or 3 seconds.