Engine skips a beat

[thomasstuart]

Hello, I have a 2000 Nissan Frontier, V6 manual speed transmission 135k miles. Recent belts changed, fairly new plugs, regular maintenance done.
My question is: when I'm in 2,3, and 4th gears, the engine seams to skip or miss a beat... not sputtering or back firing. no engine lights on, temp. gauge is normal, etc. It's worse going up hills and more acceleration. Idles only slightly rough. Any suggestions?
Thanks,

[B18cEG]

Do those engines have coil packs? maybe worth checking them out, could be a little faulty at certain times, also im sure there fine but make sure the plugs are the correct type