Question: how to do the integration with variable?

The following commands can be plotted by maple 13&15,
but can't be done by 16.

restart;
x1 := sech(sqrt(a)*t); x2 := -sech(sqrt(a)*t)*tanh(sqrt(a)*t);
g1 := proc (t) options operator, arrow; int((int((eval(x2, t = tau))*sin(Omega*tau), tau = 0 .. u))/(eval(x2, t = u))^2, u = 0 .. t) end proc;
plot3d(subs(a = 1, g1(t)), Omega = 0 .. 3, t = 0 .. 1);
g2 := proc (t) options operator, arrow; int((int((eval(x2, t = tau))*cos(Omega*tau), tau = 0 .. u))/(eval(x2, t = u))^2, u = 0 .. t) end proc;
plot3d(subs(a = 1, g2(t)), Omega = 0 .. 3, t = 0 .. 1);
I1 := int(x1*x2^3*g1(t)*g2(t), t = -infinity .. infinity, numeric);
plot(subs(a = 1, I1), Omega = 0 .. 3);

and the last two commands take a long time to plot and I can't wait,
how to plot it in less time?

thank you very much in advance.