Hot weather seems to bring on 2 types of repair, the major being the cooling system, and followed closely by transmission. Don’t panic if you are having transmission problems chances are it may only be a solenoid or low fluid. Be sure to check your owner’s manual on how to check the fluid level they vary greatly. If the fluid is OK but looks dark I would take it to a tranny shop and have it scanned, if no codes are present then have them drop the pan and check for debris. There should be NC for a code and road test. If there are no debris in the pan then have them change the filter fill with fluid and recheck. Be certain they use the correct fluid for your vehicle. For instance some Chrysler transmissions are so finicky about fluid that only Mopar ATF Plus 3, Type 7176 will perform correctly. If the fluid looks extremely bad then and only then would I consider a flush and fill, usually if it is this bad there will be some debris in the pan and most tranny shops will be able to tell if you need a rebuild or not, bad news is they usually will. Many shops will install a rebuild and repair your transmission when they have time for a flat fee. This is a simple way to go and usually is about the same cost as a rebuild. I don’t recommend used transmissions due to the cost of R&R and you don’t know how long they have sat around. Remember don’t panic until the service writer comes at you with a clipboard and smile. Also at any time you have the option to stop the process and get a second opinion, or estimate. In many parts of the country prices vary greatly locally, they are about the same where I live except for one shop that tells you up front they may use some used parts, I have used them on several occasions and have had good luck, but shops like this are hard to find.