1998 S10 heat wont work sometimes

When you say the heat doesn't work are you meaning the fan doesn't blow out hot air or just doesn't blow out any air? if it blows cold air sometimes when you have the heat on then it could be poor circulation through your radiator or a weak water pump. make sure your radiator is clean on the inside, not too corroded or nasty inside. You can buy a radiator flush thing from wal-mart or any auto parts store. See if it helps.