I think the answer is in your question:

See "whether flushing the tranny and changing the filter would help ... " and if not "start looking for a used tranny ..."

Be sure to empty the torque converter as well. Check the vacuum connections to the vacuum modulator on the side of the trans.

Hope this helps,

Haasman

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'03 E320 Wagon-Sold
'95 E320 Wagon-Went to Ex
'93 190E 2.6-Wrecked
'91 300E-Went to Ex
'65 911 Coupe (#302580)