Car jerking going up hill, then slipping out of gear and reving up
 

2002 Mazda Protege ES, 2.0 liter 4 cylinder, automatic transmission. 150,000 miles.

Car is jerking/slipping when I go up hill or when highway grade rises or when ever the car is in a laboring state. The car begins to jerk as I speed up around 20 MPH and continues as I am coming up to speed. The transmission seems to be slipping out of gear around 39 MPH. When it slips out of gear, the engine revs up to 3000 RPMs. It continues to slip and rev up until I let off of the throttle. I can let off the throttle and drive ok for a while as long as I don't have to go up hill. I'm not sure if it gets worse the more it heats up or not, may just be my imagination. This has been happening for a month or so. I checked the transmission fluid. I was able to put a quarter (1/4) of one quart of fluid in the transmission until the stick read full. Initially, when I pulled the transmission dip stick out to check the fluid, I noticed it was not seated right. It was not sealed, all of the way in the hole. I had the battery replaced a while back. I wonder if the guy accidently pulled the stick up when he pulled the battery out to replace it. The stick is right next to the battery. I wonder if over time the fluid evaporated out of the dip stick hole? I don't see any transmission fluid leaks any where. I recently moved from San Antonio, TX to Tucson, AZ, been here a years or so. It's hotter and dryer here. The fluid looks normal as described in the literature. Adding fluid did not seem to help the existing problem.

I have also wondered if my plugs, plug wires are in good condition. I will be pulling them to check today.

I have also wondered if my fuel pump is the issue or maybe something is dirty or clogged somewhere in between, before it finally gets to the engine. Maybe dirty fuel or water in the fuel? I hope its not the fuel pump, have to pull the tank out on this one to replace it.

I recently bought a transmission filter and fluid to change out the fluid. After 150,000 miles I figure it is overdue. Never been changed. My bad.

It's really been a great car so far. No major issues.

Any ideas, anyone? Thanks! :)

Sounds like to me a major issue! Time for a new transmission.
Install an external tranny cooler.

READ: How often should the automatic transmission fluid... — Yahoo! Autos

not so fast. Try replacing the filter and fluid. I am sure the filter is clogged and a new filter may help the situation.
Also change the plugs and wires/coils.