2000 transmission

i have a situation. for a while now i have had a problem with my transmission. It would only show its head on trips longer than an hour or so. the tranny would slip out of 4th and into 3rd, but if i shut off the power and turned it back on the problem would be resolved. however this weekend instead of slipping to just 3rd it slipped to neutral and wont go into gear. i had to have it towed home. I have a 2000 s-10, 2.2 4 cyl. it has 140k on it. i'm hoping its something simple and i dont need a whole new tranny. any advice would be extremely helpful. thanx.

Fluid level and condition?
Has transmission ever been serviced?
Have you had it checked on a good transmission scanner?
Engine running good?
Any codes?