stalling

Hello. We own a 1994 Infiniti Q45.When we take it on the highway and it gets up to around fifty miles per hour it stalls.Then it takes several tries to restart. Its like its having a hard time turning over.Under fifty miles per hour it runs fine, except sometimes when you come to a stop it runs really rough like it wants to stall, but bounces back to normal within like 30 seconds. please help,Rick