Bad automatic transmission

I have a automatic turbo 350 and lately its been giving me problems. About once or twice a week ill start up the truck and it shifts hard in reverse and then shifts hard in drive. When driving itll stay into first gear gear. Ill put it in park and shift it in and out of the gear. Itll take about 5 or 10 minutes but then itll shift smoothly into drive and shifts great. Idk what the problem is. Im sure the transmission is bad but I thought id get someones opinion before I go buy another one. Could it be the flywheel by any chance or is it the transmssion itself? Thanks