Toyota Premio D4 3S-FSE Engine

how are you measuring temp ?

Most thermostats i know of dont open until 82 degrees c So 78 would be not be normal running temp

It might be that there is a vacuum leak. As long as the engine is cold, the vehicle runs at a richer mixture setting. But as the engine reaches normal temperature settings, the system will go to it's leaner settings. That excessive leanness MAY be your problem. But it should be easy to check for. Spray some flammable penetrating oil around the manifold area. If the engine changes how it runs when sprayed, that is your vacuum leak. good luck.

good test method but be carefull in case engine backfires and ignite spray, but yes a good diagnostic tool

That also becomes a learned procedure. I have learned to shoot short bursts, and wait for a reaction, before spraying more. The eyebrows should grow back in about 6 months.