I have a full warranty preowned RX300
At 20 MPH car will drag when I release accelerator.
I experience a sudden drop in speed similar to a mild breaking. Is this caused by the weight of the car or is this an engine problem?

I experience the same thing as well. It feels as if the brakes are slightly engaged everytime I release the accelerator at low speeds.

Humm... I have a '99 and it wasnt till after this past winter did I start to notice this too.

I sometimes watch the tach. when it happens. When I let go of the accelerator, the tach. drops really low then goes back up. I know this is normal to some degree, but I feel a very noticable jolt every time. We should inquire about this with the dealer

Are you sure it's not just the over-run fuel cut off ?
When you close the throttle, the fuel is usually cut off ... to protect the cats i s'pose.

If you're describing what i think then i get something similar on the IS200 ...
Say you approach a r/bout, lift off throttle, fuel cuts and you kinda jump into deceleration.
Then a slight touch of throttle, fuel switches back on and you jump into acceleration.

Specially noticable when it's warming up ... ie the idle speed is higher ...
Then, this on + off is happening about 1.5-2.0k rpm so 'jerks' are quite sharp.
Crap isn't it - and quite embarrassing - makes you seem like a rubbish driver !
Yes Yes, OK, so it's simply exposing the truth .... Ha Ha very funny

Does that sound like what you're experiencing ?

I think that this is the Flex lock that is used on the RX. It locks up the torque converter on initial overrun then releases it after a time. Obviously in the lower gears at 20mph the engine braking will be more noticable.

Forgot to mention:- if you don't like the Flex Lock-up I believe you can ask your dealer to disable it.

It's designed to save wear on the brakes