This question is quite common on our site as having a number of jobs in your career may make things difficult to remember when you started at a particular company for example. You have stated that you have tried Social Security and the IRS which are two that I would recommend to start with. Other ways to possibly locate your work history can include:
1. Credit reporting agencies will have background information on file. You can request a free report each year by checking with the site
2. Lexis-Nexis also has a service that offers a background check that can be sent to your attention. Details and form to fill out can be found here:
3. Search also through older W-2 statements as they will give you start and end dates for various employers you have had.