We have a 1978 IH 1486 on our farm. We mostly have Massey Fergusons and we are not real familiar with IH tractors. We found the tractor at a farm that was fixing to close the barn doors. The price tag was too good to pass so we went and looked at it and bought it. It had a few minor things wrong so after we bought it and got it home, I went and got a few parts that i needed and tore into it. I had to replace the seals in the hydraulic clutch piston, replaced a seal on the pto shaft, new A/C belt, headlight, changed all fluids and filters....then headed to the field. This tractor has 4000 hrs on it and runs like scalded dog. After getting our crops in the ground I took the tractor to our shop and cleaned it all up. A few days later buddy of mine called and asked if I could come over to his farm a few miles away and help him get the last few acres of his crops in before the rain set in. Ready to go show off the new Red Iron (new to us) I jumped in the 1486 and headed to his farm. I pulled out on the road in 4th High and let r dig. I had a car approach me on the county road so I throttled back....when I throttled back the high/low range stick jumped out of gear. I didnt pay much attention to it the first time so I just put it back in high and took off. The same thing happened again. So I started to look into it a little. I noticed that every time I back off the trottle while the stick is in high range, the stick will jump out of gear...no matter what gear its in...1-4. So I tried it in low range, 1-4, and it would not jump out of gear. I checked everything before we bought it, however, I only drove it around the old farmers barn yard just to make sure everything was working. Now, I am not an idiot on tractors and we do all of our maintenance ourselves. I know that it could possibly be two things. A shifter fork or clutch adjustment. I am just looking for some help from any of you IH experts before I pull it in the shop and split the tractor in half....or go adjusting the clutch. Any help that anyone can give will be gratly appreciated! Thanks!!!