Kitchen Sink Water Heater?

We got a tankless water heater last year to cut down on use of fuel and be more efficient. We were hoping that it would also address the problem we have with our sinks; it takes a long time to get hot water to them, especially in the kitchen. The tankless heater turns out to be much better for the bathroom sinks which are right above the unit, but it still takes a long time for the warm water to reach the kitchen, and once it's hot, if I cut back on the flow to save water, it turns cold again. We were looking for a way to provide hot water more efficiently to that sink but the units we've looked at don't seem to do the trick; they look more like instant hot water devices for making tea. Any ideas?