95 GMC Jimmy tranny problems

ok i have a 95 jimmy 4.3l 4x4 with the 4L60E, i knew it had a small leak and was getting ready to fix it. Two days before i was going to fix leak it started slipping getting on the interstate so i drove the 2-3 miles to my parents and checked the fluid it was down a little so i added fluid and got it to the hot mark. When i left my parents it drove fine with very little slipping for around 4-5 miles at which point it really got bad the rpms would shoot up and then it would catch and go but i could only run about 35 mph. When i would stop it then go again it would shift out through 3rd. Saturday i changed the filter, fluid, and pan seal (which was leaking). Took it for a test drive up the road and really had no problems. I let it sit over night and on sunday drove it and once again same problem with it slipping and not going into over drive.If i start out with it in overdrive it will only shift through 2nd gear but if i start in drive it will shift through 3rd. Not quite sure where to start because check engine light has not come on at all. Any suggestions please
Thanks in advance

Internal problem almost definitiley, a trans scan at a trans shop will tell you if the solenoids are operating properly, after that it is coming out.