Alan
I've realised that the Odesolve_Test file I sent on 18 July had a different spring stiffness from the results I added on 21 July. I reconciled this on the attached updated version of Odesolve_Testb. x(t=.011)'s now match exactly. On my chart 0.4918 is the maximum velocity in the range 0 < t <.011 which of course doesn't match your v(t=.011), but my v(t=.011) is now a perfect match. You've boosted my confidence big time in my results, and for using Mathcad as an alternative. Much appreciated.
Ken