Nintendo 64 Console Freezes With some Games

Hi just want to know uf someone has fixed Ninte do 64 Motherboards before, ive encountered this problem numerous times, about more than 100 units wñhave passed thru my hands with same problem, it freezes with certain games, we use a diddy kong racing game to test all trade in units costumers bring and most freeze with it. Ive tested numerous jumper packs and different diddy kong racing games just in case, we also cleaned the connectors. Ive read numerous times it must be the ram but would like to know if someone has the cure. thanks guys.