my 91 did the exact same thing. it was very bad plugs and dirty injectors iv also herd that a clog in the vacum hose right b4 the pcv valve will cause that as well

 

Thanks. I parked it till i get time to check the plugs. I checked all the vacuum lines and they were fine.

 

Another thing, if it's only doing it periodically... Check your gas tank. Mine did this sometimes, although I don't think it specifically did it in 4th. Took about a cup of dust/dirt/crap out of the gas tank which the fuel filter was catching and clogging on (downshifting solved it because of sloshing). Haven't had a problem since then.