900 rpm idle & 14-15" of vac suggests a non-std cam, ie a cam with increased duration/overlap.
That requires more timing at idle.
This may not make sense, but PLEASE try it anyway, easy to do, takes 5min.
Engine idling, increase initial timing to 20-22* [ actual amount is not critical ]. Idle rpm WILL increase. Reset idle rpm & mixture screws. Test drive to see if flat spot is improved/cured...
