Dropping RPM in D

Honda Jazz 2006 Automatic Mileage : 234000 km Last Oil Change : 227000 km

The RPM keeps dropping while drive is engaged. The issue isn't there in Neutral gear. The RPM drops from a steady 800 to 200 and the whole car shudders to stay alive. The car has switched off now twice while the car was idle in Drive mode.

I had this same behavior at 230000 km and the clutch soon burnt out and I had to change the whole gearbox (used). This behavior is coming back again so I'm concerned that the root cause might not have been fixed with the changed gearbox. Any advice before I see my mechanic again?