Electrical even though you tested the batteries... Keep in mind the GM terminals can and I have seen them burn out in the battery. Top posts test fine as this is the easy button for a battery tester hook up. Yet the GM terminals were high resistance or burnt clean open.
Battery with no reserve left: tests fine for a moment but voltage and current drop like a rock after like 10 seconds of high drain.
Bad connection/wire getting hot: slows down cranking as it gets hotter.
You loose 1 of 4 brushes in the starter and cranking speed goes way down. I have a pic in the carnage thread of a starter arching in the brush area badly. Again high resistance in the starter from a worn brush can give you fast then slow spin results esp. if a brush is sticking.
Drop the fan belt. Did something lock up like vac pump, idler, etc?
You can always spin the engine over by hand and see if there is something binding.
Battery with no reserve left: tests fine for a moment but voltage and current drop like a rock after like 10 seconds of high drain.
Bad connection/wire getting hot: slows down cranking as it gets hotter.
You loose 1 of 4 brushes in the starter and cranking speed goes way down. I have a pic in the carnage thread of a starter arching in the brush area badly. Again high resistance in the starter from a worn brush can give you fast then slow spin results esp. if a brush is sticking.
Drop the fan belt. Did something lock up like vac pump, idler, etc?
You can always spin the engine over by hand and see if there is something binding.