it sounds like a return line leak. just for $hits and giggles, you might try bypassing the metal return line that runs under the intake. you'll have to make you a hose manifold that will connect the two return lines from the injectors and the IP return together and out to the tank return line in back of the engine. but if you've already eliminated all the supply lines from under the intake, that will isolate everything under there except for the IP. you might be able to conjure up some windshield washer barb unions for the injector lines or something, just don't leave those as a permanent fix! other that cutting the steel line just before it goes under the intake and running a hose over the top.
just thinking outside the box