Don't rule out the radiator. I replaced mine a few years back due to a catastrophic leak, and discovered drops of ATF in my coolant recovery tank off and on for months afterwards, even after several coolant system flushes (but my tech never de-oiled the system). Fortunately it has finally stopped.

As you know, ATF from the transmission circulates through the radiator so it is not unusual that a bad radiator will allow comingling of coolant and ATF.

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Steve
'93 400E