Thanks for the help, all. The air gap has been adjusted and definitely wasn't the problem. Cylinder compression was 80-90psi initially. Last time it happened a day or two ago, the battery wasn't able to push past the first freeze point, but when I attached my jump pack to the battery, it cranked like mad, so maybe it is a battery problem after all? The battery spec calls for #U1R-3 300CCA, I've got a 350CCA (430CA) in it now, and apparently that's about all the juice I can get out of this form factor with a standard battery. So I guess it's battery replacement time, then back to business as usual with trying to get the darn thing to crank over when the new battery starts to weaken.