Very unusual to find this with the agitation routine you describe. The streaks align with the sprocket holes and originate from one edge and fade towards the other edge. This is common when the film is given little or no agitation (thus my assumption that you used stand development).
Time for some controlled experiments:
- try the same film in a different camera, then develop with the same routine
- try a different film in the M6, then develop with the same routine
- try a different film and camera, then develop with the same routine
- try developing with the agitation routine recommended by AGFA (continuous agitation for the first 30s then 5s agitation every 30s)
Also make sure you tightly control all other variables to ensure no light leaks when handling the film. Good luck, and let us know what you find.