Had that problem once and it ended up being a couple of bad(although brand new)spark plugs.Grab your timing light and see if everything is steady.My next guess would be either vacuum leak/lean condition or LACK OF POWERVALVE.You have a carb(well..two)not an injection system.You need a PV on any street car,And I believe most racecars.There is no way for the main jets to handle every rpm,every change in engine load,every change in manifold vacuum.Remember,the rating on the powervalve is the amount of vaccuum required to hold it closed.When the vacuum drops,from where you step on the throttle,it opens to allow the fuel mixture the richen temporarily to correct the situation.Kinda like a pump shot.Just like Carter cars had stepped needles to handle different throttle positions,Holley's used powervalves.Plugging the back never hurt me on anythinf,but plugging the front won't do ANYTHING but cause you to have to over-jet to add the "lost" fuel.Then your just wasting very expensive gasoline.I never ran a front PV.Always heard they were junk.Got to thinking"then why did Holley even bother?".Started playing around and picked up about 3 mpg.Lots of tunung to be done with those little boogers.Whatever vacuum your engine idles at,get a PV rated 2" below that.Holley reccommends 1-1.5,but with a rowdt cam and uneven idle speed,it will cause the valve to flutter causing rich idle that idle mixture screws won't fix and a wandering up and down idle that nothing will fix.Carbs need powervalves,it not a barrel valve setup.Good luck and let us know something Eric