2006 rm125 shelled out again and still wouldn't run right

I bought a 06 rm125 and the motor was clapped out no power and bogged/cut out on top and then tossed the rod. Tore it down and the rod bearing was shot and the left main bearing was eating into the case half. So I bought a complete top and bottom end kit and a new left case half from an 03 different part numbers but it said they interchange. I started it ran it for 5 minutes about 1/4 throttle to seat ring and then went and rode it around for maybe 5 minutes and it started to bog/cut out like before so I gave it maybe half throttle and it ran alright then all of a sudden heard a thunk and it shut down. Put a scope down the spark plug hole and the piston looks fine but the rod is shot. Not sure about the crank and main bearings yet. Any ideas on why it bogs and why it blew up almost immediately in the exact same way