Transmission Failure problems of the 2017 Toyota Tacoma

Transmission fails to start off in 1st gear especially when under a load such as a/c on during a hot day. Also constantly shifting from 6th to 4th then holding 4th for a short duration then shifting to 5th and immediate back to 6th then starts the process over again. I have verified this is the case with even new 2018 models the dealer and Toyota state this is normal operation. The issue is we never know when it is going to occur. We can drive from light to light and sometimes it leaves in first from a complete stop and other times it does not. This is dangerous when making unprotected turns in front of traffic. The most recent occurrence I was making a left turn in front of traffic and the vehicle refused to accelerate at the normal rate with the throttle near the floor it eventually grabbed first gear and began to accelerate normally just in time to avoid a collision. As stated this issue is more apparent in the summer heat of las vegas nv. Fellow consumers suggest that I invest in a 3rd party "tune" that resolves this erratic shifting behavior but the cost is nearly $800 and I don't want to void my manufacturer warranty by applying a 3rd party software repair but this is described by fellow owners as "the only fix". The issue is well documented in Toyota owners forums as well as other social media platforms but Toyota has their head in the sand after release of their 2nd revision to the shifting program over a year ago. Also, it should be mentioned that due to this constant erratic shifting (cruise control emphasizes the issue) we have never achieved anything close to the advertised mpg either we average 14 city 20 hwy 16-17mpg combined. 20 hwy mpg is achieved without cruise control, when using cruise control that hwy mpg drops to 18.