1992 Toyota pickup 2.4 transmission in my truck doesn't want to shift

1992 Toyota pickup 2.4  4-cyl mileage: 95,000. My auto transmission in my truck doesn't want to shift from 2nd to 3rd and into o/d without lifting my foot off the gas. The fluid was brown and I had a mechanic put in a new filter with a partial fluid change and also added Trans x. This has done nothing for the problem. The problem seems to be much worse in the afternoon than when I first start it in the morning. Any ideas?