I have a 2004 A6 with 100,100 miles on it... and I can definitely tell the difference in how the suspension feels when its cool vs when its hot. I live in South Dakota, so I drive mine in -20 F weather. I have a shudder at higher speeds in my car, its done it since my dad bought it new. Stealership can't figure out what causes it. But yea - at warmer temps, suspension will likely feel looser. As for the transmission, you should probably change the fluid and filter - 98 was a long time ago. :)

2004 Audi A6Q 3.0L
Dolphin Grey Metallic