G430 RAIM failure

I went up for currency with my (former) CFII. We were flying back to
San Carlos (KSQL) and on the GPS 30 approach. We were cleared to depart
OSI at a heading of 140. Norcal informed that they are going to fly us
7 miles past AMEBY. They flew us about 3 miles east south east of AMEBY
when they had us do a left turn direct AMEBY. I was perfectly
established and 2 miles out from the FAF the Garmin 430 went into
approach mode. I had the needles perfectly centered about 1 mile from
CUZUP (FAF) when I was kicked out of approach mode. We stayed at that
altitude as we are very familiar with the approach and terrain and we
skimmed one very thin cloud for about 5 seconds and then went fully
visual. During that time my CFII did a RAIM check. I thought it passed
but maybe I'm wrong. On the ground, he was unsure why we were kicked
out of approach mode. He thought it might be because we were more than
25 (?) degrees off of the AMEBY-CUZUP course but then again it
definitely went into approach mode before that. We were kicked out
while theh needles were dead centered.

Any ideas?

Gerald