When moving between markers close enough and fast enough some timer seems to trigger the current tooltip to be hidden. I.e. the current tooltip just flickers in and out even though the cursor does not hit mouse out on the marker. I simply removed all use of timers and set fade out time to 10ms to fix this.
I'm really not a JS-person, but my suggestion is that you create a new DIV for each tooltip instead of reusing the same over and over. Or perhaps a stack of DIVs if you worry about memory.