AC Blower Turns Off For 30 Seconds/Temp Differential

It's 80 degrees in my house with 58% humidity. Time for some AC.
Comfort 90 Max (Rheem) with Central Air
2 questions:
1 - With the fan set to "On" and the temp set to 70, I turned on the AC. Here's what it does:
The blower will run for about 20 minutes (I didn't time it) and then shut off for about 30 seconds. It does this continually and it's been doing it for at least a couple of years. I first noticed it last summer but I can't honestly say whether or not it's been doing it since the unit was installed 5 years ago.
It only does this when the AC is on, not when the heat is on or when the blower is set to On just to circulate the air. It's only when I'm using the AC does the blower take a break.
Is this normal?
2 - I checked the temperature differential between the return trunk and the plenum just above the AC coils.
77 in the return trunk, 64 in the plenum, except when the blower turns off (see above). When the blower turns off, the plenum temperature drops to 59. After the blower come back on (~30 seconds later) the plenum temperature goes back up to 64.
What do think about that differential?
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An update:
Now that the house is at 71 degrees, it doesn't seem to be cycling the blower. I just monitored it for 40 minutes and it hasn't turned off. That's longer than when the house was at 80.
The temp differential is 69 - 57.
Shouldn't it be higher, like around 15+ degrees?
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Every system I've seen, if you set the fan to on it stays on continually

Seems on the low side. Somewhere around 18F is more typical. But it depends on how much air is moving, ie the air handler size, the speed it's actually running at, together with the cooling system capacity.
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Does the compressor shutdown as well?
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Don't know...will have to check the next time it happens.
As I mentioned in my update, once the house cooled down, the blower stopped cycling and stayed on continuously, as least for the 40 minutes or so that I was monitoring it.
Since this was an overnight thing, I didn't have a chance to spend hours monitoring the system to get more specific details. Maybe this weekend, if it's hot enough I can get more data.
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You will have to actually monitor the compressor and the blower, to see if the blower stops whenever the compresor starts under heavy load, like when it is reallyhot. WHen it is not as hot, the compressor may not start under as heavy a load and thus somehow not shut off the blower.
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