problem starting after 20min drive

[Barry]

That was my T3 with the issues.

There are several things that will cause this

• key switch - high probability
  
• starter motor - unlikely
  
• solenoid - medium
  
• bushing - high
  
• voltage drop - high
  

Can you use a multimeter?

See if you are getting volts to the solenoid.
If not - it is likely to be the key switch
Worthwhile changing anyway.
Can be a bitch if never changed as they have a shear bolt that needs an easy out for the first time.
As long as an allen key has been put in, its easy after that

Starter motor itself rarley fails.
The bushing in the gearbox housing is likely as the solenoid jams against this
A sharp whack can provide relief, but it would need to be replaced.

The solenoid itself can also fail.

The hot start relay kit, in my new opinion, is a good option.
Relatively cheap first option and a good add on anyway.