727 wierdness

I'm hoping it's not a sealing problem. It was driven a like 10 miles after the rebuild and it worked. Then it sat in the car for around 12 years...it has been moved up and down the road a little over the last couple years...it did not have the kickdown on it but it went into 3rd and was not driven but maybe 5 miles in the last couple years. Now with kickdown on and car finished it goes right through the gears the first few times you drive it then all the sudden starts not wanting to go into third. It has a B&M shift kit in it.