Motor problem on Miele S380 vacuum cleaner?

The motor on my S380 vacuum cleaner has developed a fault. After switching on, it runs at a fraction of the revs that it should do for about 1/2 a second and the revs then reduce for another 1/2 a second making it sound as if the motor will cut out completely. However, the revs then return to the earlier, slightly higher position and the process then repeats. I have checked for blockages & tangles, changed the filters, but with no effect.

Any suggestions. If I have to remove the body, what type of screwdriver do I need?