Esii help

[Connor_s12]

My esII has this strange problem I have not been able to diagnose. The viewfinder gets stuck closed every once and a while, and it's not the mirror being stuck up like I had originally thought/seen in my other film cameras. It's a horizontal shutter this is a separate mechanism, it blocks out the viewfinder when the Shutter (like the mirror does as well. Every once and a while it will get stuck closed and the only way to open it is to shoot 10+ frames and it It slowly opens little bit by little bit. Any ideas on what is causing this and how to fix it?