Amplifier output goes haywire with load attached
Start with your output stage. It would appear you intended to create a class-a source follower.

For the first case the most current your output driver will sink is 
$$ I_{sink,max} = 12V/6k = 2 mA$$

Driven into a 40 ohm resistive load the max negative swing you will see is -2mA * 40 = - 80 mV. Similar to what your simulation shows.

Aside:
You have an NMOS diff-pair driving a second NMOS differential-pair.
Also no global feedback.