Transmission Issues

So I noticed the other day that my transmission seems to be jumping from the gear it’s supposed to be in to one higher. So if I’m driving at 38 MPH (who actually goes 35?), normally I would be at about 1600 RPM. If I stay off the gas for a few seconds, it bounces back and forth between 1000 and 1500. Normal idle in gear is about 700, out of gear is 900. While the transmission is under power, it behaved normally. It’s only if I coast for more than a few seconds. If I tap the gas it goes to normal for a few seconds, then back to jumping around. The car still shifts through all the gears just fine (tested on the freeway: using cruise control makes the issue not occur). This issue is especially evident when I’m going down a hill riding second. It still seems to hold the car back like it should, no lurches like if I downshifted after getting up to speed. There is a little shuddering when on mostly flat ground and this happens. (Almost unnoticeable, so I could be imagining it). 227k miles on the clock, original everything, regularly serviced. Fluid is clean, full.