yes and i don't know. that's why i'm asking a lot of questions.
i'm not real familiar with your unit. is it a hydro? the next time it locks up take the plug out of it and see if it pulls easy and watch for any oil sputtering out of the plug hole. if it pulls over easy with the plug out immediatly after the plug is removed then you getting oil in the cylinder like del9175 suggested. this is most likely from very worn or broken piston rings.
i didn't ask if this locking effect happens after it has set for some time? after you do get it started will it start the rest of the day ok? when running does it run ok without ant blue smoke? if it locks up after a few hours or over night it could be you needle and or float bad in the carb and could be filling up the cylinder with fuel. that's one of two reasons i asked if the oil level was correct.
we all should figure this thing out for you. i know i asked you to do a lot of things and maybe we can eliminate most of them and find the problem.