Every so many days or weeks "cough" / "sputter" when starting?

Every so many days or weeks "cough" / "sputter" when starting - happens less if I press on gas lightly while starting (this has been an ongoing problem for 2 years). If it does start to have problem, pumping the gas will usually prevent it from stalling. If not, then I have to wait 2-3-4 minutes and after repeated tries, will start - this is all usually accompanied by strong gas odor. Have had a few things tried while having other issues repaired but as its an intermittent problem, the shop says no way to determine what is really going on. Because a friend mentioned full injectors as a possibility, had them cleaned recently but about a week later problem showed up. Any ideas of what to try / tell the shop to test for?