I have an '85 GN w/ the stock (as far as I know) transmission. Lately when hot (warm), it doesn't seem to want to shift back up. It stays down in D or OD and won't come up unless I really slow down Any suggestions as to where to start looking for problems? When it's cold it doesn't seem to have any problems, but after driving for 30 minutes in the warmer weather it starts. Thanks.