Loss of guidance, West of Delhi India - 21st May 2009 at 00:00

Requested track shortening whilst in the Delhi control zone. Made left turn to next waypoint now avoiding overflying Delhi. Radar advised that my heading was 50 degrees to the left of desired heading. Made corrections and reverted to yolk mounted Garmin 296 for additional guidance. Established on track the 530 gave a new heading of almost 60 degrees to the right of radar vector. 296 agreed with Delhi radar. Both Garmin's showed same data around 25 minutes later in the cruise and were then reliable to the destination Pathankot. Tracked today over the Himalayas and both Garmin's found reliable.