92 Land Cruiser vapor lock or ???

When the outside temperature is in the high 90's our Cruiser
loses power after driving 30 minutes or so. It will still idle but
the motor will not rev in gear. You can kill it and re-start it and
it will run fine for 5 or 6 miles then the motor starts losing power again.
Any ideas what might be causing this problem?

Thanks
Mike

Do you hear any rattling from your cat? If so then it is probably plugged.

head gasket???