I have a 99 GC with the 3.3 engine and 4 sp. auto transmission. On these hot days the transmission will lag shifting from 3rd to 4th gear, the rpm' s run up then it completes the shift. Only does this when fully hot. Any ideas?

 

Come on guys, someone must have some ideas. Thanks

 

Change the fluid with the recommended fluid and pray it works.

If not you could try to put a lower temp thermostat on the coolant system; I use an OBD II and noticed on every car I use it on that the transmission and engine temp are always within 5 or so degrees of one another after the vehicle is warmed up and driving. All of the vehicles I noticed this on share a radiator and fan for transmission and coolant. I don't have your vehicle so I don't know if this would matter but it's a cheap thing to try.

After these two things you could try a rheology modifier like slick 50 or some other snake oil and hope it saves it.

A secondary transmission fluid cooler would work wonders if you weren't already slipping, but I've never in my life messed with them and I don't even know how they work. I just know every truck with a towing package has a special cooler for the transmission other than the radiator and fan.

Here is a hit I got from google. It's an exact fit for vin code R Dodge GC 3.3, but there were two 3.3 in the drop down menu on the site. It is only $54 and I now realize your current radiator or transmission cooler might just be corroded inside and limiting flow, and thermal efficiency.
https://shop.advanceautoparts.com/p/...B&gclsrc=aw.ds
Install video...this guy made a real mess installing. You can do this better than he did. He really didn't need to cut his bumper frame. He could have put the cooler up higher or down lower.

 

Change your fluid/filter. ATF+4 fluid only.
If that doesn't solve it, it's likely already too late.