Stem screws came loose when riding

The stem isn't anything special. I think it's an older aluminium ITM "Millenium".

You need to

• clean and wax the handlebar/stem interface
  
• use assembly paste (a special kind of grease) on the bolt's threads.
  
• tighten the bolts to the recommended torque
  
  

Use of a thread lubricant (assembly paste) and proper torque is important. If you don't know what ITM advices one can generally use the values as specified for the bolt size. See: Bicycle Fastener Torque Specification