Might be worth looking at this thread. It might be worth checking the existing DTACK value in mon, then trying the pokew mentioned and checking different resolutions.
I have a slight suspicion that a timing issue between the A3660’s fast CPU without onboard memory could lead to bus timing issues when the ZZ9000 is busy (e.g. using Z3RAM/USB/Ethernet at higher resolutions), but this is all a little above my pay grade.