Look a the injector holders. Each should have two phenolic washers on the bolt holding the mount to the head, one between the mount and the head, and the other between the mount and the bolt cap. Those provide thermal insulation that keeps fuel from vaporizing in the injectors, which seems to occur most often exactly in the situation you describe. They are often broken, lost or discarded when the seals are changed.

If you don't have these, try adding them. You will also need to renew the large O-rings under the mounts, as the ones on there will have been over-compressed due to the thickness of the lower phenolic washer being missing.