As stated check flywheel , also check crank bering while the cover is off. I've seen sled engines drop there balls, after a cage failure does exactly the same thing. ( never seen a Lt do it but possible). If you dont find anything its possible the balancer shaft key sheared and will catch when you shut it off. Under harder acceleration it will slip again, Coming and going. I always put the balancer and flywheel nut along with clutch side crank nut with a impact gun. Ive never had one slip, ever. Don't use a big air impact though or really let it hammer it on or you will have problem.s I also clean all threads real well with brake cleaned and use RED loctipay close attention to the trans oil, a broken piece floating can cause this and will settle out when you stop but get picked up and tossed around.