Monte carlo analysis cadence
Most likely the problem is not your design but the testbench that you are using. The low gain and the high-pass behavior of your simulations suggest that the operational amplifier does not work properly, because the DC operating point is not set correctly.

You need to make sure that DC operating point is such that your input signal is within the input common range of the opamp and the output signal is not too high. With an offset in the range of a few mV and a gain in the order of 60dB or more this can happen quite easily.

The Monte-Carlo simulation is done with a fixed seed. So you will get the same results whenever you run a simulation. This helps to isolate the problem. Pick a run which is completely off and resimulate only this run. Check the DC operating point and fix your testbench.

This can be done by setting "Starting Run Number" to the number of the run you want to simulate and setting the "Number of Points" to one.

Update: After looking at the schematic I am sure that it is the testbench (or the lack thereof). As a quick fix you could feed back the ouput to the inverting input using a low pass filter with a cut-off frequency that is very low. Then you should make a real testbench.