My 94 toyota 4cyl LE speedodometer has a mind of its own.?

the car drives fine for awhile without the speedodometer working at all. Then all the sudden it comes to life and the car can't seem to know what gear it is in. Could it be a speed sensor and is this expensive and a big project. Also I have heard that there is a screen that should be cleaned. Also the overdrive on keeps blinking when it is on. No engine lights are on. ITS SO SCARY SOMETIMES. The car has 248,000 miles. I know that is alot but it purrs like a kitten and starts fabulously EVERY TIME..