Troubleshooting stock 98 4.3l...random 7500 rpm spikes?!?

http://i.imgur.com/0iqAVfz.jpg

I've been using the scan tool to try and diagnose an intermittent stumble at highway speeds in a 98 blazer with a 6 month old engine rebuild. It doesn't happen all of the time, so we've just been attaching the scanner on trips out to try and catch it. What I've gotten is really bizarre. The RPM's show as spiking 7500rpm, the injector duty cycle jumps to 65%, timing drops to about 4*, and then you get some strange readings from the O2 sensors. Then it the whole thing goes back to normal for a few minutes before happening again.

I'm assuming the RPM spike is the cause of the increased injector duty, the drop in timing, and the strange O2 readings.

My question is, what would cause that random rpm spike? The engine never obviously changes speed. You just feel something almost like a misfire and then it's fine.

I'm definitely not super experienced with HPTuners, but this just weird to me.

I can post the whole scan if asked, but since it's 37 minutes I figured a screenshot might be best to start.

Did the distributor gear get replaced when the engine was rebuilt? It's either that or the crank sensor isn't shimmed correctly.

On a side note could you post your stock tune or upload it to the repository?

I've got a 98 blazer with a 5.3 in it, trying to calibrate the fuel gauge.

Thanks!