My understanding is that Reagan lifted travel restrictions to Pakistan in 1981, however the State department issued a travel advisory:
http://dosfan.lib.uic.edu/ERC/travel/cis/southasia/TA_Pakistan1981.pdf
The reality was, however, that Pakistan was only issuing 30-day visas to the press, diplomats and those willing/able to provide large bribes. They did not want Americans there.
I travelled to Pakistan multiple times during that period. The State Department travel advisory addresses specifically airport visas, which were being issued only for 30 days. If you obtained your visa prior to entering the country, you could get a longer period of time, no bribes involved.