I got my Monte at 163,000 miles. It had many problems, after fixing most of them I am faced with a Trans rebuild. When mine started slipping, mostly in 3rd gear, I did a Fluid and Filter change. It didn't Help.
I say, spend the $$$ to have it diagnosed by a good trans shop. This way you know what it is and if it has to be rebuilt or just repaired. But it will need something!

 

So you got 200K miles more out of the trans without doing any fluid/filter changes ever? If so, I think you've got your money's worth out of it for sure.

How much longer do you plan to keep the car? If the plan is to just try and limp it along with minimal money input- I think it could be worth a shot to stick a new filter on it, and just replace the fluid that comes out from a pan drop. Maybe the filter is just all plugged up, and you can keep limping it along a bit more? Your chances of it solving all your transmission problems are slim, but its worth a try if plan B is to call the junkyard.

If the plan is to keep the car for a lot longer- I'd just get the whole trans rebuilt/replaced. You're certainly at the end of it's life as it is- and even if a pan drop/filter change worked in the short term, you're certainly not going to get another 100k, 200k out of it.

 

I've had it since '05

 

being a service manager, the first thing i would do is pull the dipstick and check the fluid condition...if its black and smells burnt, no additive, to include lucas is going to help it out..if it's brown it's just dirty...if it has a black speckles on the dipstick, that's clutch material...if it's just dirty/contaminated, you might be able to just change the filter...if the fluid has any black in it, the trans is done...with almost 300k miles on it, that trans definitely did it's job...just keep in mind that IF you do change the filter, it is possible that it may make it worse...