Now Time marker slowing up midi editing

Well , i 've been trying to diagnose this problem , but to no avail . Midi editing is quite sluggish in PRV , but ..i noticed that when i move the now time marker about 20 measures away from the measure(s) i am working on , the notes edit just fine . When i move the marker to the actual spot i am editing , the notes take several seconds to move .. never had this problem before . ????!!!