Doesnt sound right.
I recently made 6 milks with coffee followed by two hot chocolates all as fast as I could pump them out.
No problems at all with the steam.
What did you mean by "was on for two hours non stop"?
How many coffees did you make over how long?
Were any long blacks?
How much water was in ther tank when you looked?
Ive never seen the steam power drop gradually unless the low water level switch cuts the power.