2000 Sienna has check engine light on and code PO500

Hello everyone. I was wondering if anyone may be able to help me with this problem as all the mechanics that have worked on the van are throwing their hands up. They are confused and do not know what to do at this point. our van has 114,000 miles on it--bought it used. The check engine light came on and took car in. 2 places confirmed code PO500 so we replaced the instrument cluster. Also when the check engine light came on our cruise control stopped working and sometimes when we tried to go from a stop the car would not move but would rev high and then finally jolt into gear. At times right before this happens you will hear a loud knock or bang. We were told that once we replaced the instrument cluster that everything would go back to normal. Well when we went to pick up van the check engine light came on immediately and the car still slips at various times (going from first to second and then it can happen again from third to fourth). If anyone has any advice I would surely appreciate it as I have been without my car for almost 2 weeks. Thanks.