In theory you could have a global variable, say "Delay" and have something happen Every "Dealy" ms. And every time it happens assign "Delay" a random value in the given range. If it was, say between 40 and 60 ms do a "Delay" = 60-Random(21)
I have it where I make a new EVENT > SYSTEM, Time: Every X Milliseconds, 2000 (for example). Then for the ACTION > SYSTEM, Create Object, pick the object, layer 1 (for example), x coor is random(640), and y is 0.
+ System: Every 2000 milliseconds -> System: Create object HealthPack on layer 1 at (random(640), 0)