I feel, it can't be done with the timer control as it is. Rewriting the interval does not reset the internal counting of the timer. I could suggest you look at this construct if you could use it.

Code:
sub Wait(interval as single)
Dim x as single

     x = timer+interval
     while x > timer: doevents:wend
end sub
This uses the inbuilt function 'Timer' to wait while the interval has not expired. This function is useful as an in-line timer.

Hope this helps.