My transmission has started acting up lately, and before I take it into the mechanic this week - I'd like have an idea on the severity of the problem I'm looking at.

Here are the symptoms:
1. Sometimes doesn't shift into 4th gear
2. Sometimes shifts from 4th gear into 3rd gear without any reason (at highway speeds when it shouldn't downshift), and then won't go back into 4th unless I kickdown into 2nd and then let off the accelerator
3. Last night, while at highway speeds, slipped out of gear completely (I was in 4th). I had to move the gear shift down to 2 to finally get it to engage.
4. Last week it wouldn't engage into first gear when at a stop. When this happened, I had to cycle the gearshift through the gears until it engaged.

Yesterday, I quickly checked the ATF dipstick, as detailed in the manual. Looks like it is clean and at the appropriate level.

Any ideas? Suggestions?

Car has 192,000 miles on it.

Thanks,
Matt
1994 E320