its probably your settling. if you reduce your clocking by say x4, does it go back to normal again?
if this is the case then settling is your monster. to counter this you need to minimize your Ron, use wider shorter devices. use tgates as opposed to pass fets, etc
also calculate your rc time constant of your signal with the resistors in place, no matter how fast your amplifier, you cant make up for lost settling time, so you will never be able to settle faster then your signals rc value.
you can also solve for Beta (closed loop factor, the B in AB). when solving this you need to do it 2 times, look at Beta with all switches being 0 ohms, make sure you account for all parasitic caps. then do the same thing with Resistor included, this will show you the difference in your AB (loop gain), which you can then solve for your closed loop gain A/(1+A*B). (to save time you could sim for AB and B if you have the ability)
-Pb