Car gives a shutter when idling downhill

Hi, a 1999 Lexus GS300 with 115K miles. Great condition but afte I back out of driveway then put car in drive and slowly go down the hill on my street, I can almost guarantee the transmission slams into a gear (I assume its a lower gear) everytime. The fluid level is fine and the fluid doesn’t have the ‘burnt’ smell (Is that still a valid tranny fluid test?)

Any help?