Will not start? It cranks but won't run.

My husband is a delivery driver after many stops and go's the vehicle won't start. It cranks but won't run. He has to wait sometimes as long 15 minutes, then it starts. Lately it does this every day but only once a day what could that be? Also drives a little sluggish like no real power it has to build. We would like a idea because no mechanic can mimic the issue and no scans so we are at a loss, any help would be appreciated. Thank you.