tankless water heaters

Gentlemen:I have a technical question regarding the Aquastar water heater. I have installed an Aquastar about a year ago. My shower has a pressure balancing valve. The minor adjustments that this valve makes effects the flow of water through the Aquastar often causes it to stop heating the water. I understand that this might be eliminated by using separate control valves for hot and cold water supplies thus sustaining the pressure in the hot water line and eliminating the pressure fluctuations. It has also been suggested that a thermostatically controlled valve might do the same thing and also prevent scalding when the toilet is flushed. Do you have any experience in solving this problem or could you direct me to someone who does? The plumbers locally I have talked with have little or no experience with this product.RDPodulka [email protected]