My understanding is that TotalGameTime represents how long it's been since the executable opened. In your case, it's been up around 8 seconds. For your += statement, I think currently you're taking the difference of the two, and adding it to the current time...? I'm not even sure what that resulting value would be.
I think more likely, you're looking for
ElapsedGameTime (documentation on MSDN); replace the contents of the parentheses with just that to get what you're looking for:
currentMs += GameTime.ElapsedGameTime.TotalMilliseconds;
Also, I would advise you to subtract 125 from currentMs, rather than setting it to 0. If currentMS becomes 200, then it's going to have somewhat inconsistent timing.
if (currentMs > 125)
currentMs -= 125;