Help! - My tenant is calling me with a real stickler of a plumbing
problem and need help diagnosing and fixing / solving this one.

DAY1-I get a call that the Hot/Cold water surges in the 2nd floor
bathroom (duplex/upper flat) shower. The Tenant downstairs is out of
state so that person can't be flushing the toliet or anything like that
(rules out the downstairs plumbing activity).

I inspect the case: I go to the shower in question, turn on the water
to a steady warm condition, and then go to the bathroom sink and turn
on the cold, feel the bath water (remains the same). I go to the
kitchen and turn on the cold (bath remains the same), turn on the hot,
(bath water remains the same)???

I go to the basement to check things out with the water heater. The
water heater is leaking bad from the bottom and is SHOT. I replace the
water heater with a brand new 50gal/water heater.

3-4weeks pass, I think - replacing the broke water heater must of fixed
that problem.

Well I get a call from the 2nd Floor Tenant again today - " The water
is surging hot/cold (I don't get the exact behavior but get the gist of
the problem) still inside the shower" I believe the hot water gets too
hot and burns the showeree or it gets too cold one or the other.

I lived in that apartment for 9mon to 1 year and never experienced this
"surging shower water problem." Can't be the person down stairs either.

I need some help diagnosing and fixing this thing for good. Any advice
and insights much appreciated.