Fred,
Following up to what Jim Smith said, if there is a vacuum when you release the fuel cap, try operating with the cap loosened. When I bought mine it had a similar problem it would run out of power after about an hour of driving. I noticed the vacuum when I would fuel up, I left the cap loose and the car ran fine. I found the tank vent which is under the car was plugged and the hoses were soft and gummy. I replaced the hoses and cleaned out the vent. I check the tank vent everytime that I change the fuel filters. Hope this helps.
Tom
82 SD
86 SDL