> For most, the subject's complete data set comes solely from one of three scanners, but in
> a few subjects, scanner source differed across time points.
Scanner is largely a between-subject factor? In that case, it would make more sense to try the following for both of your cases:
-model "Cond+Age+ISI+Cond:Age+Cond:ISI+Cond:ISI:Age+Scanner" \
...
-ranEff '~1+ISI' \
Gang