1986 case 1845c control problems need input

my dad had a 93 1845c that had a fire. the motor was not hurt mainly the cab and wireing burned out. so we found this 86 with a hole in the block and decide to use the one we had and swap them. we have them swapped and the machine is working good but has a issue with the steering. the machine had set for up to 3 yrs that we know of due to the filters are dated 5-06. after we got the motor in and the hydrulic lines they took off it replaced the bucket and arms work great strong and do not leak back. the machine also has the auxilliary hydros. the problems is at first when you tried to start the machine it would want to take off and move forwards on its own. you would have hold the levers just right and if you let go it would move. my dad ran it into a wood pile and let go of the controls and it would just set there and spin all 4 tires. know we adjust the linkages and got it alot better and the things on the top off the valve body stopping in the center. so when you let go of the controls it will stop or kreep very little on the right side until you bump the lever back and then it will set there. when you operate it on a hard surface like a drive or hard ground it will buck and jump like a bull trying to go back and forth. know in soft dirt or sand it does not do this. my dad thinks the springs on the levers are weak do to the setting outside for years and that the little cylinders on the linkage are weak and could be alot of the problem for this. has anyone had this issue before or could offer some advice to this problem. what should we be looking for. i thought do to the setting that the vavle and pump may be stiff and have a release valve sticking or something like that. thanks for the help.