98 Lexus gs300 engine does not stop running after ignition is turned off.

What is your question?

Sounds like dieseling noise, kind of chugging. Happens intermittently and only way to stop is put the car in reverse or drive which puts load on engine. It has occurred on several occasions and when it happens the ignition key will not go completely to the off position. It happens when first starting car, so it is not from overheating.

What makes this problem better or worse?

Generally when engine is cold

How long has your 1998 Lexus GS300 had this problem?

A month or so