The technician is likely correct - the mainboard needs replacement. On a system this old, carefully evaluate the cost vs. the cost of buying a new system - repairs come with at best 90 days warranty, so you'll be spending quite a bit more (with labor) than the system is worth to repair it.
Thanks for posting. Apologies that your system is not performing as expected.
ejn63 is correct, you may need to weigh the possibilities of a new purchase rather than a repair, but if you haven't already tried, here is some troubleshooting from the Dell knowledge base you may find helpful: