My chevy van 20 1990 Wont start all the time

Hi Jenny,sounds like a problem we used to have with some of our older chevs,does the problem only happen when you have had it driving a while and its hot but once it cools down say after an hour or so it starts fine? If so it sounds like a voltage drop problem in the ignition wiring which some older chevs suffered from when the ignition wiring gets hot it builds up resistance and wont pass current well enough and as most chevs have a feed from the starter to the coil also the coil doesn't get enough current.To find out if this is the problem run a wire through a momentary on type switch straight from the starter ignition terminal to your battery positive terminal and when the problem happens leave the key in the on position and press the switch which will bypass the ignition and turn the engine over, if it starts when you do this you have a voltage drop problem,if so get your mechanic to fit a relay that is powered straight from the battery and in middle of the starter ignition feed wire which will give the starter feed straight from the battery when you turn it over from the key.hope this helps.