Solving coupled 1st ODEs with ODE45 or Runge-Kutta?

Hi there, I am having to solve a simple problem where I am equating Newtons's Second Law to the Lorenz Force on a particle: F = ma = -eE

d2x/dt2 = (e/m)*E0cos(omega*t) x(t=0)=0; dx/dt(t=0)=0 (so at t=0: posn=0 and velocity=0)

Pretty simple right? Well doesn't seem so! I am getting strange results when I solve this with my RK4 routine.

I split in to two couples 1st ODEs:

x1p = x2 x2p = (e/m)E0cos(omega*t)

where x1=x, x1p=x2, x2p=(d2x/dt2)=(e/m)E0cos(omega*t)

Is it possible to use the ODE45 to solve coupled 1st ODEs? What would your advice be?

Thanks in advance, Martin