I think I have solved the problem. I re-flashed 5.2.030 using the image manager and restored my current settings (I ended up running 5.2.039). It works fine now. I started 3 recordings making sure they used separate tuners and it was all fine - no "tune failed". I think a bug has crept in after 5.2.030 that causes the problem. It is certainly there on 5.2.034 and 5.2.039