I expect that your agitation method may be at fault. There may be a language issue, but I am not sure what you mean by rotating or tilting 360 degrees - and then turning 90 degrees for another 360 rotation. I can't quite picture that. If you are inverting the tank, we should be talking 180 degrees; in other words - turn the tank upside down.

Here's the Ilford instruction: "turn the tank upside down four times during the first 10 seconds and again for 10 seconds (that is, four inversions) at the start of every further minute to agitate the developer. Each time you invert the tank tap it on the bench to dislodge any air bubbles which may have formed on the film." (From http://www.ilfordphoto.com/Webfiles/200629163442455.pdf)

Kodak: "For an invertible tank, one cycle consists of rotating the tank upside down and then back to the upright position. For a noninvertible tank, one cycle consists of sliding the tank back and forth over a 25.4 cm (10-inch) distance. With tanks that have a handle for turning the reel, rotate the reel back and forth gently through about one-half turn at a rate of one cycle per second during the agitation intervals. After the first 30 seconds, agitate for 5 seconds at 30 second intervals. Agitation should consist of 2 to 5 cycles, depending on the contrast you need and the type of tank." From http://www.kodak.com/global/en/profe...bs/j78/j78.pdf

The operative thing here is to invert the tank (if that's possible) and then give it a good bump to dislodge the bubbles. Obviously, there should be room enough in the tank for bubbles.

Note that Ilford recommends 10 seconds once a minute and Kodak, 5 seconds twice a minute. Both come pretty close. The trick here is consistency. It may (or may not) make a difference, but which interval that you prefer is subjective.