Dethatch timing

My lawn seems to need to be dethatched. Here in the Mid-Atlantic, we are still in the hot season and my turf-type tall fescue is hanging on but not thriving.
Should I do it now or wait a month or so for the cooler weather ? Seems that less thatch might help the fescue "breathe" and minimize humidity related problems like mold.
I'm afraid that spiking or dethatching might just help the Bermuda grass thrive. I consider Bermuda to be a weed and it comprises between 1% and 20% of the yard depending on the location.
Any thoughts ?
Any ideas on how to keep Bermuda in check ? I'll never get rid of it considering the size of my lawn and the ubiquititous nature of the stuff. I occassionally kill big areas of it and overseed.
Thanks
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I wouldn't do anything physical like de-thatching or aerating to tall fescue if it's under any sort of summer stress. Wait till it cools off a bit.
I dunno squat about Bermuda.
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