'93 4Runner manual transmission problem

I just bought this 93 Toyota 4Runner ( has 90,000 miles) a month ago, but now I have problems with the manual transmission already..the third gear jumps back to neutral (unless I hold it) and It grinds going into the fourth gear. I don't get it! it only has 90000 miles. Is it unsual? Please help! What caused it? and give me some ideas how to fix it.

Thanks million

I'm no transmission expert, but it sounds like the syncros aren't engaging correctly - could be simply needs transmission oil change. Could need overhaul. Try changing to 75W90 gear lube and see if that helps. If not, better bring it to a shop.

If you want to work on it youself, try here:

http://www.drivetrain.com/autotransoverhaulkits.html

http://www.autorepairmanuals.biz/site/573683/page/267592

http://marlincrawler.com/
There is a wealth of knowledge about manual trannies at the above webiste.