1987 Chevy S-10 hesitation

Any ideas on whats going on will be appreciated, I bought an 87 chevy s10 with new head and block that was replaced at 115,000 miles, truck runs fine but seems to hesitate and cut off if left to long in neutral or park ive checked everything I could think of ranging from spark plugs to some sensors im not to experienced with these trucks but all an all they seem to better than anything ive had even with this problem so an advice would be appreciated thanks