C1233 & C1234 OBD2 error codes - Ford Focus

When I plugged in an OBD2 scanner into my vehicule (Ford Focus 2003 ZTS 2.0L, VIN on demand) a few months ago, I got a C1234 (Speed Wheel RF Input Signal Missing) error code. When I plugged in my OBD2 scanner a month ago, I had both C1233 (Speed Wheel LF Input Signal Missing) and C1234 error code. When I plugged in my OBD2 Scanner last night, I only had the C1233 error code. What can cause these alternating (on/off) error codes, and what can I do to fix it.

Of course, my ABS and TC (traction control) yellow light are both illuminated on my dashboard. My neighbord has already checked the ABS connectors with a ohm-meter and apparently the signal is good. Is my ABS module going on the fritz, or could my tone ring in front be damaged/dirty? He also cleaned around the ABS Connector / Bearing area but it did not make the error codes go away. Everytime that I clear the codes they come back after a few second of driving. Ford dealership wants a lot of money to even diagnose what the issue is. Should I pursue getting this repaired? Is there a common fault with Foci having this type of issue? Please help !