i had the stumble too on my 90 T with the aem. i tried several ignitors and that can work but gets expensive. You may already know this but here is how i think the stumble can be fixed and how the aem does ignition timing.
I think i got rid of the stumble by setting the physical timing with the cps to a specific spot. rotate the balancer to set the timing mark to zero TDC and insert a long socket extention into no 1 spark plug hole so it rests on the piston top. turn the crank to make sure zero tdc is set. this will make sure your timing mark on the balancer is still at zero TDC when the piston is at it highest point of the stroke.
now rotate the cps so that there is just about 1/4 inch between the rotor point and the top magnet prong (IG1?). just under 1/4 inch. this would be about 10* TDC base timing on a stock setup. i think the rotor is counter clockwise (left) of the prong.
the aem doesnt use any base timing set point. it will set the timing as inputed by the timing map and any modifiers (ait etc). the point at where the cps is position is recorded in the program as the ignition sync number. it is located in the advanced ignition options tab. mine is around 1.06. i think this is good for 89+ cars to be around this number. i think the number is a delay number to help the ecu calculate where tdc is. timing is initiated off the timing map and does not have a base number starting point.
adjusting the cps position i think helped get rid of the stumble. adjusting the coil dwell number may also. that adjustment is located in ignition, advanced ignition, coil dwell setup. mine is set to 10. you can go up and down by 2 from about 8 to 23 or so. also try differences of 1 when you get to a number that works. ie, 10, 12, 14, 13, 12, 14, 15 to try to see what works best. sometimes adjusting dwell just gets the stumble to move around in the rpm range. like from 3100 to 2800 rpm.
so set cps to about 1/4 inch, try differnet coil dwell numbers, and try different ignitors. also be sure all ground connections are clean and secure.
of course set the timing with a timing gun and use the aem ignition set up. set the timing to 0 then 15* then 5 then 10 and make sure the light moves to the correct mark on the timing cover when you do. when you set default timing in the timing setup on the aem it will ignore the map and modifiers and set the timing to the number inputed. if it is not correct use the adavance and retard tabs (course or fine) to get the timing mark to move to the appropriate number. do not use the cps to set timing. the cps will stay set to the place set above. when you hit the advance or retard tabs the ignition sync number changes to make the timing mark move in the light.
