Strad37

Hey everybody, I was hoping that someone might be able to help me out with this one. I own a Stradivarius 180ML37, serial # 482793. I have been experiencing some difficulty with sticky valves lately. I took it in to a specilist repair shop to check for dents in the valve casing, etc. because I had purchased this horn used. They didn't find anything wrong but said that they would clean it out the best the could for me. That helped it along for about two weeks before it all started sticking again. The valves will stick completely stuck and will begin to stick a couple days after I completely clean and oil them. I don't know what to do! Is there anything that you guys think could be wrong? Is there a special technique to cleaning your valves? Please, if you have any ideas do send me your ideas. Thank you.