I have a stock 2004 Silverado with 5.3 and 4l60e. When I'm on the highway the trans seems to slip until I press the gas. For instance, I'll be going 70 and the RPM will creep from 2k to 2500 or more. As soon as I hit the gas, it grabs and immediately drops to 2k again. Any ideas?

 

Does this ONLY occur at 70+ MPH?

If not... Can you repeat driving the same way, but in Manual-2?

Or if it also happens at slow speeds; in Manual-1?

 

Seems to be happening over 50. I can't tell if it's slipping or deciding what gear it wants until I'm cruising on the highway. 70 is just the speed limit on the only road I can really tell what's going on.

Also, it randomly decides to do a hard shift now and then. Doesn't seem to care which gear it's in. Maybe 5 to 10 times within an hour of driving.

 

If this seems to occur any any of the gear ranges (meaning that you have tried moving the gear-selector to some of the manual gear-ranges)...

You will need to have the Line-Pressure measured next (in every gear-range, at an idle and above idle... aprox. 1,200 RPM).

 

I'll try it manually tomorrow to see if I get the same issue in low gears and pick up a line pressure gauge.

Thanks for the help!

 

The slipping stops (or appears to) when you hit the gas because the ECM commands more line pressure in the trans. It could be that the 3/4 clutch is worn out and needs a little extra line pressure to stay engaged.
The occasional unwarranted hard shifts are due to excessive line pressure. This could be due to a sticking Pressure Control Solenoid. It could even be caused by a bad MAF giving wrong signals to the ECM which in turn commands the line pressure in the wrong way.
Have you scanned for codes? Even engine codes can be related to this.
When was the trans filter and fluid last changed?
A line pressure gauge line the ATD-5550 ($35 on Amazon) has a long hose which would let you mount the gauge on e.g. your outside mirror for viewing while driving. Besides the Idle and Off-idle pressures, the pressure in Reverse and while driving at 70 mph would be useful. We want to rule out simpler/cheaper problems.

 

I once had a customer with this same complaint. After a little quizzing, he admitted to recently removing and reinstalling his valve body and separator plate (can't remember the reason he gave). I pulled the pan and valve body and, sure enough, the pin for the 4th accumulator had fallen out and was now missing. So essentially, 4th apply oil was crossing into the accumulator oil circuit (through the pin hole in the piston), where it was then being regulated by the accumulator regulator. Only when enough throttle was applied, and the torque signal came up (increasing accumulator circuit pressure) would it hold, and as soon as the throttle was lifted and the torque signal dropped back down, it would begin to slip again. Any kind of crack in the piston would create the same situation.