I was driving up a very long, very steep hill at about 60-65 mph and noticed that my rpms started to increase as my vehicle speed started to decrease. The car basically lost 3rd & 4th gear and I drove it a few miles in 2nd gear. The car sat for a day and I tried driving it again but still didn't have 3rd or 4th gear. I coasted back down the mountain mostly in Neutral and when I got to more level roads I put it into Drive and it shifted up to 3rd. Later the same day, I drove it and the first time it tried to shift to 3rd it wouldn't go. I dropped it to 2nd then back to drive and the tranny shifted normally. The tranny seemed to be shifting fine. Today I drove the car to work (less than 5 miles) and the car shifted normally. I drove the car back home and it shifted normally. Can anybody explain this to me? The tranny (700R4 ?, '91 Camaro 3.1l) has 149k miles on it and it have been exactly 30k miles since I changed the fluid and filter. Is what happened to me called "slipping into neutral"? Do I have a "slipping" transmission? How do I describe my event? What do I look/feel for to determine if my tranny is slipping? I read that the worst thing to do to a slipping transmission is to change the fluid? But if I can't get it to "slip" again then I assume that I need to change the fluid and filter. What should I do? I'm trying to understand the problem I had and decide on a course of action to take. Any expertise or advice would be appreciated. Thanks.

It sounds to me like you trans got pretty hot an did start slipping. As the fluid gets hotter, it gets thinner and therefore can't completely lubricate and provide adequate pressure. With that many miles, you are probably running short on time. Take it easy on the trans and save up some money to get it rebuilt.

Thanks for responding 1981LT1.

I drove the car again today and it shifted completely fine and I didn't feel anything that felt like "slipping".

I guess my biggest delima right now is should I change the fluid & filter?

I would definently change the fluid and filter and see what happens.

I'd invest in an auxiliary trans cooler, too.

Yup, got way too hot and lost all viscosity. It probably couldn't move the 3-4 piston without being drained back from the pressure. The heat in the trans would cause parts to expand and get harder to push around too.

Living in the mountains one may want to see about an aux tranny cooler as 305 stated above.

Do the fluid NOW. Check it first for proper level though. You could have just slowly leaked out barely enough to cause problems. Dunno why its working ok now, but low fluid level does some VERY funky things in a 700r4.
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And yes, what happened to you is called "slipping to neutral". It feels like you accidentally bumped the shifter mid-drive into neutral. sudden RPM increase and no more GO, engine freewheels. It might "find" a gear again if the RPM is exactly right in relation to the road speed.

It probably only happened due to the mountain.

Thank you to everyone who responded.