As I'm sure you're aware, 98 (even 96) is too warm for your snakes. Where on the heat tape is the thermostat probe placed? It sounds to me like it may not be secure to the hottest part of the tape if you're seeing temps 7 degrees above your set point in a three tub rack. Also, are you sure your thermometers are accurate? What kind are you using?

FWIW, Vision brand tubs are completely flat on the bottom. I don't believe they have a size compatible with the Sterilite 1655s though. Maybe the Iris brand has tubs that are compatible. Which rack do you have?