I have experienced it before and it could be two things.  One your new coolant
level sender could be defective too.  The second is that you may have very
small cracks around the hole where the level sender screws in.  These will
allow coolant to come out at the highest pressure (when you are doing highway
driving) but not normally.  The coolant rises up high enough to get in the
sender and then must dissipate somehow.  I fixed this once by using teflon
tape and silicon on the sender and then installing it.  It fixed the problem
but you really should get a new tank if this is the case.  Also inspect the
new tank closely as I have heard of folks getting new tanks with the same
problem.
Ken Wilford
Van-Again
John 3:16