Case 1840 lift cylinder issue

[Skidud]

When trying to back blade using down pressure, my 1840 (4000 hrs) lift cylinders will occasionally "bounce", as in retract and then spring back. It also acts up when I tilt forward and apply down pressure with the tilt cylinders as well. If I don't apply down pressure with the lift cylinders first, the boom just raises off the ground. If I apply just a little down pressure for a second, the lift cylinders seem to hold alright, and I can lift the wheels off the ground and it will hold that position. However, after a minute or so of back blading, it seems to want to do the whole bounce thing if I am not diligent in reapplying a little down pressure sporadically. Is this a check valve issue? Is my loader control valve shot? I have good hydraulic pressure and cylinder speed. There is no indication of pump issues. I can fill the bucket with stone, and it will hold well, with very minimal bleed down. It only seems to affect the lift cylinders, the tilt cylinders don't give me any issues. I have another, newer 1840 (1600 hrs) and they both operate the same, except for the issue I outlined above. Any thoughts are greatly appreciated. Thanks.