So, I demoed my game on battery power.. and it sucked. Interestingly whenever windows showed a tooltip, the game ran at full pelt.. something fishy there. But anyway, I introduced a Δt (delta t for the unicode (or greek) unaware) into my calculations. The first stumbling block was that I was using the tick counter, which sucks a bit. The thing is, even though it has damn good accuracy, it is only updated every now and then. Which is a bit odd. I managed to hack it into working by doing the following: 

private void Physics()
{
    while (System.DateTime.Now.Ticks == Tick) { }
    //time interval
    long thisTick = System.DateTime.Now.Ticks;
    float dt = (Tick - thisTick) / 100000.0f;
    Tick = thisTick;
But that really feels dirty. All that hanging around waiting for the tick. - Spoon! So, anyways. After a bit of ferreting, I found the Coding4Fun: Beginning Game Development article I'd been reading, which contains a section "All about timers". In summary, I grabbed the dxmutmisc.cs file from the sample code (in DirectX SDK\Samples\Managed\Common), commented out everything but NativeMethods and FrameworkTimer, called FrameworkTimer.Start() before starting the Main loop, then did 
private void Physics()
{
    dt = (float)(FrameworkTimer.GetElapsedTime()*1000);
to get the time interval since the last physics call. The *1000 is just to get the number into something I could count on my fingers-and-toes. And, with a little tweaking of cosmological constants, all was working again.