2005 Subaru Forester Smoke & smell under hood

Hi, I have a 2005 Subaru Forester XT (Turbo) with a automatic transmission and 103,000 miles. Several months ago while waiting in traffic for a light to change, I smelled burning and thought it was from the exhaust of the car in front of me, then saw smoke coming out from under the hood. Several attempts by a repair shop to figure out what is causing the smoke have not found the cause. The Timing belt has previously been replaced. It only smokes at certain, now predictable times, usually after about a half hour of driving, etc. Is there something I should be looking for or directing a mechanic to look for? It make me quite nervous to drive it, knowing that it is getting so hot that it smokes. Seems possible it could catch on fire. Thanks for your time and input.