Ok sender is in normal spot, I took it as you had it in passenger side head or something. You can let the coolant circulate at idle, yes it will push some out and you will have to top it off before you put the cap back on. Only reason I brought it up is the temp spikes you are describing point to blockage. I cant think of what would cause blockage/ temp spike, then go away other than a bad stat or major air lock somewhere.