I've got a 98 monte with a 3.8l v6 and auto trans. A few days ago it stopped shifting. just wouldn't leave first. Let it sit over night, shifted fine in the morning. but then stopped again after a while. I changed the filter and it was fine again for the night. but by morning had the same issue. also, when the trans stops shifting the speedo stops working as well. not sure what's going on. any ideas?