What can cause a watchdog failure in an INDRA MSSR?
1) What I expect is that there is a transmitter (or multiple) on 1090 MHz to the west of the RADAR that does not conform to the ICAO standard. I would start by looking at the decoded replies that the MSSR is receiving (not in ASTERIX format, but raw datagrams format. I believe the INDRA MSSRs can provide this). This may give a hint as to what the cause is(e.g. a certain 24 bit address range, certain downlink formats only, received power level, etc.).

2) Are there any military bases to the west of your RADAR? Or RADAR manufacturers? A transponder manufacturer? It may help to contact them. (Likely they will deny any experiments are going on, but the problem may suddenly disappear. It happened before). 

3) Atmospheric tunneling may cause a specific transmission path only to exist under certain weather conditions (e.g. inversion layers) . Can you find any correlation between the interference and weather conditions?

4) Do you have multilateration systems covering the same area. The multilateration system may be able to locate the source. 

If nothing else helps, you may have to install an array of 1090 MHz receivers and multilaterate the source.