99 Transmission problem

I have a 1999 GMC Suburban(5.7) with 173k miles. When the outside temperature is above 80 degrees F and I have been driving for about 30 minutes, I get a hard/firm shift from 1st to 2nd. It shifts to the other gears normally. When the temp is below 80 degrees, I can drive all day without any problems. Any clues for what may cause the hard shifting on hot days?

I have a 1999 GMC Suburban(5.7) with 173k miles. When the outside temperature is above 80 degrees F and I have been driving for about 30 minutes, I get a hard/firm shift from 1st to 2nd. It shifts to the other gears normally. When the temp is below 80 degrees, I can drive all day without any problems. Any clues for what may cause the hard shifting on hot days?

How many miles and has it been serviced?

I would get a transmsson capable scanner on it and also pull pan and check valve body springs and spacer plate area for problems.
Post back any transmisson or engine codes.

I have a 1999 GMC Suburban(5.7) with 173k miles. When the outside temperature is above 80 degrees F and I have been driving for about 30 minutes, I get a hard/firm shift from 1st to 2nd. It shifts to the other gears normally. When the temp is below 80 degrees, I can drive all day without any problems. Any clues for what may cause the hard shifting on hot days?

when the transmission slips the PCM commands high pressures which causes the hash upshift. have the transmission checked out. pressure testing etc. could be overheating or valves. could be major work also.