Monarch snowplow hydraulic pump on tractor

Sorry for a non-tractor question, but the snowplow is on my tractor and the snowplow is run off a older monarch dyna might hydraulic pump. I hope some one can help me?
Here are the symptoms: The pump is activated by rods or cables. The plow has can be moved left and right as well as up and down. THe hydraulics work the right and left of the blade, but it does not lift the blade. The pump seems to be working, because one of the bleeder holes on the top of the control box shoots out oil. I have checked the fluid level and it appears to be fine. There are times where The hydraulics lift the blade and it will hold it up and another time it will lift it and slowly sink down. Another problem I notice about the control lever is that it gets stuck when ever I try to activate the up and down lever. It is like some thing is bound inside. I have taken the return line to the reservoir off and proded into the control box with a piece of wire and that seems to free it up momentary. Would ice build up in the control box...We have had some warm weather up here in Northern Minnesota the past couple of weekends. Thanks for any answers

Dan

if you think freezing is possibly the problem; warm it up inside shed and then put a couple bottles of the best heet or similar deicer you can find in the hydralic tank and run it..if ice is the problem this will clear it up--I too am in Mn and have mositure problems in hydralics once in awhile

Mike