Once in the nether world it crashes game and deletes Nether Portal

i have deleted several worlds because each time I make a nether portal and enter the Nether the game slows until it crashes. when I restart the world I am still in the nether but my portal is gone and once again it slows until it crashes.

I normally see messages in the logs and I appologize for not copying them stating Java could not keep up ignoring ect.

Next world it occurs on I will add a file log