Some weird stuff happened going up my freaking hill the other day, my truck lost gear. I don't know how this can happened with an auto and the th engaged. I have been a little out of it, and I was saying to myself why the hell did I put it in neutral in the middle of the hill, and looked down and everything looked normal on the evic. Gonna change fluids, I guess it is official I will have to drop the pan and just look for metal. Never had anything like this happen.
If it only happens on a hill, it could just be low on fluid. If you're on an incline and the fluid level is a low, it could be that the pump can't scavenge enough fluid and loses line pressure. That would cause it to act as if it were in neutral. I've had it happen on a TH400. It may not be your problem, but worth a good look.
Checking transmission fluid properly is more involved than most people care to deal with. This is the procedure as/per Alldata:
FLUID LEVEL CHECK
Low fluid level can cause a variety of conditions because it allows the pump to take in air along with the fluid. As in any hydraulic system, air bubbles make the fluid spongy, therefore, pressures will be low and build up slowly.
Improper filling can also raise the fluid level too high. When the transmssion has too much fluid, the geartrain churns up foam and cause the same conditions which occur with a low fluid level.
In either case, air bubbles can cause overheating and fluid oxidation, and varnishing. This can interfere with normal valve, clutch, and accumulator operation. Foaming can also result in fluid escaping from the transmission vent where it may be mistaken for a leak.
The transmission has a dipstick to check oil level. It is located on the right side of the engine. Be sure to wipe all dirt from dipstick handle before removing.
The torque converter fills in both the P (PARK) and N (NEUTRAL) positions. Place the selector lever in P (PARK) to be sure that the fluid level check is accurate. The engine should be running at idle speed for at least one minute, with the vehicle on level ground. At normal operating temperature (approximately 82°C. or 180°F), the fluid level is correct if it is in the HOT region (cross-hatched area) on the oil level indicator. The fluid level will be approximately at the upper COLD hole of the dipstick at 21°C (70°F) fluid temperature.
NOTE:
Engine and Transmission should be at normal operating temperature before performing this procedure. After the fluid has been checked, seat the dipstick fully to seal out water and dirt.
Start engine and apply parking brake.
Shift the transmission into DRIVE for approximately 2 seconds.
Shift the transmission into REVERSE for approximately 2 seconds.
Shift the transmission into PARK.
Read the transmission temperature value.
Compare the fluid temperature value with the chart.
Adjust transmission fluid level shown on the dipstick according to the Transmission Fluid Temperature Chart. NOTE:
After adding any fluid to the transmission, wait a minimum of 2 minutes for the oil to fully drain from the fill tube into the transmission before rechecking the fluid level.
Check transmission for leaks.