That appears to be a direct acting poppet type relief valve in a screw-in cartridge. I couldn't find enough info to decode the full part number, but that helps a little.
You should know that _any_ pressure control valve is just abut guaranteed to oscillate, absent some internal damping.
The internal damping mechanism may not be obvious; it's often a closed cavity with clearance its only opening, or perhaps a small orifice.
The important point is that the cavity may not bleed itself right away, or ever, and if it has air in it, it won't work.
Try pre-bleeding the relief valve by immersing it in a cup of oil in a bell jar, then evacuating the jar and holding the vacuum until the bubbles stop. Then put the cartridge back in the assembly.
Mike Halloran
Pembroke Pines, FL, USA