game lag at around cycle 19

my game has started to lag with 8 people at cycle 19, checked my computer and cpu is only at 50% capacity, intel i5 and gtx 970 with 8gb ram and a solid state hard drive. closed and restarted game and worked ok? anyone else had similar lag issues? and i did try all the speed settings all lagged, pause and un-pause didnt solve the problem either.

the same thing happened on my 7 year old laptop but thought it couldn't handle the game so tested it on my new pc (above) same thing happened. but since the restart on the new pc havent had any issues and colony is much bigger.