Sometimes, when my car is fully warmed up, it will up-shift harshly. Why? on 2004 Buick LeSabre

It does not always do this. It may go days before it does it from one time to the other. Only when it gets warm from a lot of stop and go. I had the filter and fluid changed 3 months ago and it seemed pretty good for a month, then in the second month it did it again, only for a couple of days, then back to normal. Today it did it again. Just can't figure that out.

Asked by for the 2004 Buick LeSabre
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1 more answer
I use BG Trans Conditioner with my trans service. Just changing the filter leaves half or more old fluid still in the transmission, so the conditioner helps with the old fluid. Changing it again would not hurt, you can get rid of some more old fluid.