Sounds like it's temp related. When the ATF is cold there's no problem, but once the trans warms up you get the trans failure. Could be the temp sensor that kicks it into trans failure mode, or could be an internal leak because the ATF has thin out once it get hot.

The cheap fix would be the trans temp sensor, and that can be verify with a scan of the EGS module when the trans is hot. The expensive fix will be a complete rebuild.

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