I have a toyota Townace KR-42 (1998) with a 7K-C engine (Automatic transmision). About 3 months ago I had a Radiator block and had to change the thermostat, gasket, and repaired the radiator. However, after I ran for some time the engine started to shiver in a way that the whole vehicle vibrates. Every day for the 1st 20 mins or so the vehicle runs smoothly but after the temperature meter raises a bit it starts to shiver and when I accelerate a bit (on neutral) the shivering stops & when I let it go the shivering sometimes stops but then again it starts all over again when I accelerate to move on the road.

Any suggestions to why this is happening and how to resolve it?