96 elantra timing belt broke... we replaced it and put new heads....

does your car have a IAT sensor installed (idle air control) a faulty sensor will definitely cause that,vaccum lines / broken,missing, leaking, even still could be the belt timing on the odd chance the crank or cam sensor is not getting a sequenced signal

Check for vacuum leaks, check the coolant level is correct, check compression make sure is all even, is there any service engine soon light on? There is a company that independent auto repair shops get their information from, this same information is available inexpensively for people that work on their own cars.. The information is year make and model specific, covering repair procedures, torque specifications, fluid capacities and specifications, service bulletins, component locations, wiring diagrams ect.... Alldata is very easy to navigate http://bit.ly/AllData_Repair_Manuals_Online Not sure how far their information goes back but Hyundai as a car manufacturer put a lot of free technical information and workshop manuals on line for Hyundai owners. Check out www.hmaservice.com lots of great information.