If you are truly losing the rear end without any "spirited" driving than you have something bent or loose in your suspension somewhere. As odd as this may sound check both front and rear. If nothing appears out of whack make sure you check the tightness of the bolts/nuts. I had a front stabilizer bar link come loose and had to be retorqued. You can find the torque specs in a maintenance manual (I bought mine off of amazon.com).
I had a front wheel drive car that one of the lower rear suspension members had been bent and it had tendencies like you mentioned. I would be going straight and at the right speed/frequency the rear would want to swing out. It was weird!
If it is coming around smoothly, meaning you don't notice a jolt, I definitely think something is bent.
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