Willy,

Thanks for the reply - I understand the limitations of Windows - in fact I
am seriously considering an embedded hardware solution with a REAL RTOS
(Forth) for this application ;-)).

I still would however expect the timer period to remain accurate, even if
some ticks start late (as they were blocked), I would expect these to
recover, but the effects appear to be cumulative :-(( - when one is delayed
the next also appears to be delayed - hence my question about when the timer
tick actually gets reset.

I'm also used to embedded systems for many years and would always ensure
that the first thing an interrupt handler often does is to reset its source.

Thanks agin though.

Graham