Unwanted output signal jitter

[skimask]

Now the +/-20% range is adjustable and will probably never see 20% and may be reduced to some narrower range in the end, but until then, I'm starting at +/-20%.

If I was you, I'd kill that 20% idea and stick with something less, like maybe 5%. Like I said earlier, 20% is the difference between stoich and melted pistons, especially at high rev's, heck 20% is the difference between slobbering rich (say 12.5:1) and the lean side of detonation (12.5:1 - 20% = 15:1).

will I need to have a PIC device with 4 timers to be able to trigger off 4 inputs to "sample one, build the next"?

One timer, keep track for 4 start/end value pairs, do the subtraction math on the pairs to find time differences.

In keeping with my philosophy of keeping it simple...
Jerry.

I agree 100%+ with every one of those comments...and with that agreement and $1.25, you might be able to buy a bottle of Dew!
But seriously, dead on, let the hardware do the work for you.