I drive a '99 Sedan with manual tranny. Sometimes when I'm in 3rd gear when I'm driving it will randomly pop out into neutral. It doesn't do it all the time but every once in a while. Does anyone know what might be causing this?

 

your syncro is going out, or if it does it in all gears it could be a bad motor mount.

 

No its just 3rd. So that means my 3rd gear synchro is going out I guess? Would it be worth it to get it fixed? Or find a used transmission.

 

Depends on how much a transmission shop would charge you to pull it and replace it vs if you can find a decent replacement somewhere.

 

I found a transmission from the same car I have on ebay for $200 with about 30k on it. How hard would it be to replace? Drain the transmission fluid, remove the linkage, unscrew the mounts, then put the new one in and do the reverse?

 

drain fluid
eh pull axels (lower ball joint removal aint fun)
remove shift linkage
support engine (rent a hoist, its the easiest and safest way)
unplug wires
remove clutch hydrolics
unbolt trans and carfully drop down

that is the basics but buy a haynes manual its a good investment and it will have a better instructions then i can give you

 

I've got a haynes, I didn't check it for that yet though. Yeah I didn't think about the fact that part of the engine is supported by the transmission/transmission mounts. I'm guessing you also have to drain the clutch line.