Had this EXACT issue on my 2gmf13 with the KM2a transmission. The original manual had a misprint and told people to run SAE30 NON-detergent oil. Non detergent will leave varnish
in your cone clutches that adds up over years until you get slippage and intermittent failures to engage.
Easy fix. Get the proper SAE30 HD (high detergent oil) and change it every few hours of run time. I did, one about every 4hrs of engine
and each time and it came out cleaner and cleaner until a nice gold. The transmission shifts PERFECTLY ever since. That was about 1000hrs ago. Hell I've made it down the Erie canal
, finished my 3mo in the Bahamas
and am sitting here in eleuthera for a weather
window to WPB after the weekend. Zero fails engagements zero slips.
Anyone who isn't sure go into your engine
bay and find the metal stamped tag on your trŗnny. Hope that fixes it for you too! I was pricing repowers when i fixed mine for about 30$ worth of common Quaker state 30HD.