That is normal for the stock 5.0. Sounds like you need an H/I/C setup. The biggest bottleneck is the lower intake on the stock 5.0. You can get it ported by a member here named tmoss http://www.tmossporting.com/ or buy an aftermarket one. For an aftermarket one and you wanna maintain your bottom end stick with the edelbrock or cobra/gt40/explorer(all 3 flow within 5 cfm of each other). Obviously the Ford Explorer one could be found for pretty cheap. After the intake the heads are the next restrictive thing. AFR and Trickflow make probably the best in the reasonable price range Twisted Wedge being the cheaper. As for cams, get a custom one but if you are on a budget the TFS stage 1 is a great off the shelf cam.
Kevin
Kevin