Possible to replace Thunderbolt port?

MacBook Air 13" Mid 2011 running 10.9.1. Thunderbolt monitor working consistently well. But when I use Thunderbolt > VGA cable connections to various monitors/projectors it is unreliable; sometimes signal repeatedly dies, sometimes monitor doesn't detect at all, while other laptops work fine. Have tried different cables. Evidence of slight corrosion in one corner of the port.

Questions:

1. Does it make sense that Thunderbolt monitor would work but that there would be issues with Thunderbolt>VGA monitor connections?
2. Is it possible to replace Thunderbolt port without replacing entire logic board?
3. Do USB>VGA options work with Macs; if so, do they carry audio?

Many many thanks for any advice.