911 1970 HOT (ELECTRICAL) START PROBLEM

Vehicle Information: Model: 911T 2,2; Year: 1970; Body Type: Coupe; Engine (type, size, modifications): 2,2 STOCK; Total Mileage: 148,000.;

Car starter occasionally won't crank- specifically after a long(45 min. +) freeway drive, with hot outside temperatures (90 deg. +.)Engine oil temp is between 190 and 210 deg.As soon as car is shut down, it can immediately re-start. However, the problem arises if I attempt re-start it about 10-15 minutes after it is "shut-down."Car can re-start about 30 minutes later,when cooler?.Cold starts are flawless.I have a new ign. switch (electrical), new batteries, new Bosch OEM starter.After doing research, I understand the electricaltroubleshooting procedures, i.e., ensuring (8 amps?) gets to terminal #50 on the solenoid.My car is 34 years old and is a "driver"- some tech articles suggest a starter relay "booster" add-on between the ignition switch and the solenoid. I am hesitant to perform this potential "quick fix" as the caveat is that it doesn't solve the specific problem such as faulty (-)ground, older wiring, low conductivity/high resistance from hot wires/starter components, etc.Does this sound like a classic "older car" situation? I am tempted to get an electrical specialist familiar with "older" 911 models.Thanks and regards!