MF135 perkins engine seized ??

Need some help. 68 MF135 perkins gas engine. Been running fine. Oil filter and oil changed 30 days ago. 10-15 hours on it since then. Oil level full but it is still like new after 10-15 hours. Dragging my gravel road for about 30 minutes. everything fine. Leave it running and get off to talk to neighbor. We both hear it sound like it went under a load for 1-2 seconds and then back to normal. I jump back on and start dragging the box scrapper again. All is good and then it sounds like its under a load as it dies. 3-5 seconds and its dead. Starter wont turn it over. Tow it to garage. Put jump starter box on it, still wont turn over. Starter is engaging. Watching crank pulley and it doesn't budge. Have wife pull me with truck just enough to pop clutch to break it loose. Pulley rotates a little. Try starter again and engine locks back up. Buy bore scope and remove plugs. Plugs are perfect tan and no damage. Cylinders look fine, no gouges, scratches, burn marks. Tops of pistons look fine. Starter bumps pistons a very little bit each hit. Ideas of what might be binding?
I don't know if there was oil pressure or if it was running hot. Radiator was full when i checked it back at garage. I expected to see scratches or a few busted ring parts but it all looks good. 30 days go i replaced the distributor and cleaned the plugs. Checked compression for the heck of it and had 150, 125, and 150. Little concerned about the 120 but it was running good. any help is appreciated.

That is exactly how my 65 acted when it spun a rod bearing ruining the crankshaft and rod. Rebuilt the whole engine and it runs like new. Try removing the starter and see if you can turn the engine backwards with a bar on the flywheel teeth. Mine freed up backwards but don't try to run it because if it does run it will seize again and do more damage. Don't ask me how I know. One of my MANY COSTLY LIFE LESSONS. FUNNY HOW YOU NEVER FORGET SOMETHING LIKE THAT.

Hello,
To add to what Phil has said it will likely be the number three rod as it is furthest from the oil pump.

DavidP, South Wales

Thanks for the reply. I think you are dead on. And I have learned a lot of those lessons the hard way as well!

That falls in line with my 65, it was #4 rod bearing.