Transmission goes in and out of reverse/1st in fractions of seconds, but does not slip when running

[GTV6]

My 2000 ES has 146 k miles , have done a slow drain on the tranny once and shifts through the gears in normal fashion . When I select "R" on the shift lever, the tranny will go in and out of gear rapidly for fractions of a second then engage. 

This is an intermittent condition , first occurred 3 days ago , then again the following day and now normal operating for last 2 days. Auto technician could not find any faults, agreed that there is no slipping in the tranny when running, he ran it hard and shifted thru the gears.

Has anyone experienced this and were you able to diagnose the problem and what was the resolution.  Again the tranny does not slip or flare up when driving. It makes me think we are looking at some electronic sensor on the tranny going faulty or is it the shift lever sensor if one exists. Appreciate any help.

Dodge