Usually if one of the pumps is dead you can do a flow test by taking the feed line off and either jumpin the relay or having someone crank ( be careful for spilled gas) use a container to catch it. You will see a real good flow if good or just a medium flow if bad. Such problems like yours can be caused by stopped up fuel filter or even bad air filter as well as alot of other things. In order to really rule out fuel system a pressure gauge would be handy here. We have also seen bad ignition components breaking down upon load do this also. A scope would be handy for this also to look at secondary to see if there is any breakdown.
Mercedes Technician 7 Years (retired to Hyundai)
2000 Dodge Durango
98 Mazda truck