Bought this Husqvarna ZTH6126 a couple of weeks ago, seller said the engine was locked up. Turned out to be a seized PTO clutch bearing. Replaced the clutch, and it fired right up. Thought it was my lucky day. Mowed about 15-20 minutes, and the engine started to sputter, then died. Obvious fuel issue, and apparently heat related. Let it cool a few minutes, and off we go again, for 10-15 minutes, then repeat. Got out the fuel pressure gauge, hooked it up, let the engine cool, and tried again. Fuel pressure at 39 psi. Started mowing again, and after 10 minutes, pressure started to fall off gradually, down to 12 psi, and the engine died. Pinched off the fuel return line to dead head the pump, and no rise in pressure. From my automotive experience, I thought I had a bad pump. New pump arrived today. Installed it, and headed out to mow. 15 minutes in, same problem! Fuel pressure tapered off again, and no fuel returning to the tank. Let it cool a few minutes, and we have flow through the return again. I don't now what else to check. It has the problem on either fuel tank. I let it run for nearly an hour this evening at a high idle with no issues, but as soon as it's under load mowing for 10-15 minutes, it dies again. Must be heat related, but I don't have a clue how. Any help would be greatly appreciated.