Can't stay on LNAV, starts turning away from track

Hey guys. I'm doing a flight from FSIA to OMDB in the NGX, and maybe about 30 min. after passing the Equator from South to North suddenly the airplane started turning to the left away from my track. LNAV was still engaged. I switched to HDG to get it back in track and once again engaged LNAV. Seemed to work, but 5 min. after passing the next waypoint it did a 90 degree left turn for some reason. I haven't touched the IRS's, but the FMC is saying: Scanning DME Fail. But that shouldn't really make the airplane do 360 degree turns or fly 90 degree away from track..

I have been flying in the NGX for hundreds of hours now and this is a first.. It's not that S-turn thing. I have had that issue as well long time ago, and this didn't look like it was that.