jumping out of 3rd could be as simle as a worn detent plunger and spring, or could be something more like gear and bearing wear.
there is a row of slot headed plugs downt he side of the tranny that hose the shifter detents.. you can unscrew with a big flat blade screwdriver and pull the plug out and use a maglet to fish the plunger and spring out. 3/R is the top gate and gets the most use.. kinda like a shuttle.. wears the most.
if it looks real worn, here's a trick. 4/5 is the bottom gate.. gets used the least, if at all.. pull it's spring and plunger and swap them.. if that solves the issue, buy a new set and repalce them all.. or at least the worn one you swapped into 4/5.
if that don't do it, you got 2 more options.
1, full trans rebuild ( might be cheaper to buy another trans? )
2, redneck-repair... 3"" spring with 2 loop/hook ends with about 2" lead on each side of the spring, a sheet metal screw and washer.
put shifter into 3rd gear, slip 1 spring hook end around shifter nec right under ball, stretch a bit till it's under a few pounds of tension, and then mark a dot on the lower dash cowlingwhere the other loop end of the spring could be made to touch.. remove spring from shifer, install sheetmetal screw and washer to hold spring to coulwing, and hen you use 3rd, slip the spring over onto the shifter.
I got one 850 that if you decel/accel going down hill it will pop out of 3rd occasionally. it's an ugly work tractor used a few months a year to unload hay, and not worth a 500$ fix... so it got the spring.
also to think about.. is this an 850 or an 860 you ar elooking at.. both are 5spd, but the 860 is live pto / 2 stage clutch.. more valuable.
does it have power steering?
loaders generally wear the front end a bit.. expect that.
does the hoe have a subframe? or is it a 3pt or does it lack a 3pt?
is this really an 800 model.. or a later 4 cyl industrial tlb with a heavier front axle that simply has had it's trans swapped out from an 850/860?
how are the tires?
does she start easy?
hyds work.. all hoe and laoder controlls work? will it lift a load?
an average 8xx with a few issues and a how and laoder could be from 1500-3500 depending on your region and it's actiual condition and the answers to those questions above.
loadder style/design makes a difference as well as the hyd source for the laoder and hoe...
soundguy