I have an 89 seaswirl sierra cuddly classic. 302 with a one year old sei 116 outdrive. Bought a new shifter with the outdrive. It has been working good for a year , shifter adjustment has been great. Started not going into neutral when pulling back on the shifter. Adjusted again and I can get it shifting perfectly with the water muffs running in the driveway. Put it in the water and drive it, it won't come back into neutral. Once you the get shifter into the neutral position it will stay in drive for about 30 seconds then pops into neutral. If you shut it off goes right into neutral. Adjusted and adjusted but can't get it to change. Any ideas?