Just had a 4l60e installed in my c5 that was supposed to be in perfect working condition. Got it in and every gear seems slushy with the 2-3 shift flaring up and hitting the Rev limiter. Does anyone know what could cause this? Tune has been checked and is good

 

As in used?

What did it come out of?
Anything done to it?

 

These are symptoms of either a worn out trans and/or one with low line pressure. Low line pressure might be commanded by the PCM or be a problem inside the trans.
Problems with the MAF or TPS can cause the PCM to command too low line pressure.
The correct way to diagnose this is with a trans pressure gauge connected. While pressure at idle in Neutral/Drive might be around 60 psi, when you blip the throttle it should shoot up near 200 psi.
If the pressure is good, then your trans is simply worn out.
If the pressure is low, it would be best to connect a scanner which shows the commanded line pressure; if the command is high but the actual pressure is low, then it could be a bad Pressure Control Solenoid in the trans or just a worn out trans. If the commanded pressure is always low, then especially the MAF and TPS should be checked; a good scanner can help with that too.
Also, a 2->3 shift flare is nearly always the symptom of a worn out 3/4 clutch, the most common problem with the 4L60E.

 

Ahh... The 2 - 3 shift has always been a big complaint with this transmission all the way from it's earliest days in 1982 as the then named THM700R4.

From stock to wildly modified this transmission can be plagued with 2 - 3 shift timing issues... even on perfect/ fresh/ new builds.

I have found few builders to not only get a solid, well timed 2 - 3 shift, but to maintain it, and not lose it, as the trans starts to wear...

For myself personally, I prefer to block the 3-2 control valve in the closed position, and omitting (BLOCKING) several of the 3-4 clutch circuit passages in the separator plate so that a much more simplified version of the 3rd accumulator and 3-4 clutch circuits can create a repeatable and fast firm up and down shift (2-3 and 3-2).

 

I wouldn't bother with a pressure test. You find good pressure theres something wrong internally. You find bad pressure theres something wrong internally.

Take it out and have someone who is not supposedly anything but a good transmission builder and have one built to work.

This is assuming you have the right type and amount of fluid in it and ran it long enough.