Car is surging

This is kinda of hard to describe but when going down the road it feels like its braking off and on, its just slight but I can feel it. Seems to only do it in 4th gear so hopefully this will stop when I put in my new transmission in a couple weeks. The brakes are fine I checked them all out. Any thoughts?

Its just started doing this in the last couple weeks but it was worse tonight. I think it might possibly be my clutch slipping(I know its worn out) and getting replaced when I put in the new trans. Like I said its kinda hard to describe. Its not really violent enough to be a surge, its more like its just slowing on and off, just enough to make my head go forward slightly in between. The plugs/wires are about 6 months old frpp wires and ngk plugs, the coil packs I have no idea how old they are, I will replace them with Screamin' Demons soon. I don't think its engine related because its running good. The rear end is fine, I checked it out just to make sure (I rebuilt it myself with 3.73 gears) so I know whats going on there. The car does; however, have 164xxx miles! I'm building a forged PI engine and I'm polishing the heads and probably some light porting, I should have it done in a few months. I will list the mods in my sig so you all know what has been done. Thanks for the help

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