This sounds like gear train damage, either in the front or rear...Basically, as gear teeth start stripping, a few the pieces get wedged in between the ring gear, planetary carrier pinion gears and/or sun gear, preventing a mesh so will 'lock up' the transmission for a split second or two then will clear...They will also clear if you shift from reverse to forward and vise versa. The front planet is particularly crucial to movement as it's splined into the 3-5-R drum which is mated to all of the hubs directly or indirectly and those hubs are the link between the front and rear of the trans. Hard to say which gear group is going bad (or perhaps both are) but if you had to have one to bad, it's preferable that it would be the front gear train as it's a lot less costly to replace than the rear Ravigneaux style planetary carrier and compound ring gear set as those can be upwards of $600 to replace w/good used or over a grand for one with a billet drive plate.
My video, 6L Diagnosis and Troubleshooting goes into the above in detail and touches on symptoms of a failing gear train; if you're curious you can check it out.
