[clinebarger]

The solenoid could have a weaker magnetic field when it gets hot, Or the coil could go completely open with heat.

I've been there & have several T-shirts!!!

A low amp clamp & a scope would prove out my theory. Or disprove it & you could still have a mechanical issue with the solenoid.

As Bigdav points out...You could have TCC valve sticking, But I doubt it given the symptoms.