Starts sputtering, tries to stall while driving

My truck sometimes starts sputtering, tries to stall while driving. The RPMs go up but it will not accelerate when applying the gas. Not all the time. There's no rhym or reason to it I can drive around for hours then all a sudden it's like the engine bogs down RPMs go up but it will not accelerate i've done tune up, oilchange, fixed vacum leak, just put fuel filter on the old one had very dark gas in it could there be some underlying problem there. Also valve cover gasket is leaking slightly from the vacumm problem I think any suggestions are appreciated