Spark lines from o'scope

[A70Duster at aol.com]

A bit of help from you all out there.  I attached my inductive pickup from my
timing light to the coil wire and hooked it up to a port on the o'scope
(Chan. A).  Got the nice spark lines and coil ocsilations like I should, but
as RPM changed, so did the grouping of spark pulses.  

Here is the request:  Is there a simple and cheap ( < $30) circuit that I
could attach to Chan. B that would adjust for a certain about of pulses
across the screen, then start over (like when #1 fires).  It should work from
idle to max RPM (6000).  

A triangle wave generator would give me the sweep, but at a constant
rate...???..

Any help greatly appreciated.

See ya,

Mike