You probably have a poor needle valve. Here is a list of valves acceptable for our usage that I have seen recently posted on two forums regarding planted tanks; I have ordered the S series Swagelok off of eBay recently and am putting it in my rig.
The Fabco and Ideal seem to be the most popular.
Swagelok 20 series needle valve, soft seat stem. Part number SS(B)-20R*
Orifice: 0.08"
Cv: 0.09 (at 2.5 turn full open)
Swagelok O series needle valve, soft seat stem. Part number SS(B)-OR*
Orifice: 0.08"
Cv: 0.09 (at 8 turn full open)
Clippard MNV-3/4 series needle valve, part number MNV-3* , MNV-4*
Orifice: 0.07"(MNV-3), 0.067"(MNV-4)
Cv: N/A (at 4 turn SCFM is 1, Cv is under 0.03 before 4 turns, after 4 turns, Cv spike)
Ideal valve 52(brass)/54(stainless steel)--2 series, Part number 52-2-*, 54-2-*
Orifice: 0.0625"
Cv: 0.082 (at 20 turn, 22-24 turn full open)
Hoke 2300 series metering valve, 8 degree angle stem
Orifice: 0.062"
Cv: 0.075 (at 20 turn full open)
Parker Hannifin NM series, Part number *-NM?-*-*-*-*
Orifice: N/A
Cv: 0.055 (at 14 turn full open)
Fabco NV55/FC55 needle valve, Part number NV-55, NV-55-18, FC-55
Orifice: N/A
Cv: N/A (approximately at 0.04, The SCFM is 1.7 at 10 turn full open, similar to swagelok 31 series)
Parker Hannifin HR series, Stem type 5, Part number: *-H5?-*-*-*
Orifice:N/A
Cv: 0.049 (at 15 turn full open)
Parker Hannifin NS series, Part number *-NS?-*-*-*
Orifice: N/A
Cv: 0.042 (at 14 turn full open)
Swagelok 31 series metering valve, Part number SS(B)-31R*
Orifice: 0.062"
Cv: 0.04 (at 10 turn full open)
Parker Hannifin HR series, Stem type 4, Part number: *-H4?-*-*-*
Orifice:N/A
Cv: 0.032 (at 15 turn full open)
Swagelok M series metering valve, Part number SS(B)-?MG*, SS(B)-?MA*
Orifice: 0.056"
Cv: 0.03 (at 9 turn full open)
double pattern, dual handle
Cv: 0.026 ( at 9 turn full open)
Hoke Millimite 1300 series metering vale, 3 degree angle stem.
Orifice: 0.047"
Cv: 0.024 ( at 18 turn full open)
Parker Hannifin HR series, Stem type 3, Part number: *-H3?-*-*-*
Orifice:N/A
Cv: 0.021 (at 15 turn full open)
Ideal valve 52(brass)/54(stainless steel)--1 series, Part number 52-1-*, 54-1-*
Orifice: 0.0313"
Cv: 0.019 (at 20 turn, 22-24 turn full open, approximate Cv 0.008 -0.009 at 10 turn)
Parker Hannifin HR series, Stem type 2, Part number: *-H2?-*-*-*
Orifice:N/A
Cv: 0.014 (at 15 turn full open)
Hoke Millimite 1300 series metering vale, 1 degree angle stem.
Orifice: 0.047"
Cv: 0.011 ( at 18 turn full open)
Hoke 2300 series metering valve, 1 degree angle stem
Orifice: 0.062"
Cv: 0.01 (at 20 turn full open )
Parker Hannifin HR series, Stem type 1, Part number: *-H1?-*-*-*
Orifice:N/A
Cv: 0.007 (at 15 turn full open)
Swagelok S series metering valve, Part number SS(B)-SS* , SS(B)-SM*
Orifice: 0.032"
Cv: 0.004 (at 10 turn full open)
double pattern, dual handles
Cv: 0.001 ( at 10 turn full open)
Hoke Micromite 1600 series metering valve.
Orifice: 0.031"
Cv: 0.0008 (at 18 turn full open)
Parker Hannifin HR series, Stem type 0, Part number: *-H0?-*-*-*
Orifice:N/A
Cv: 0.0004 (at 15 turn full open)