My 1999 BMW 740i start to act up only in hot days while driving in heavy traffic (lots of stop and go) the transmission get stuck in low gears. Engine temperature and all other gauges are normal no warning lights or problem are indicated. The problem will go when I turn the engine off for couple of minutes and let it cool.

Get your AT Fluid checked asap. Low fluid level, blockages and expansion can manifest into these symptoms. Procedures for DIY check of AT fluid level can be found on e38.org website.