The primer pump could have a bad seal. I have had this happen to me in my 240d and the same thing happens.

Also, The injection pump may have an internal problem, however, you need to follow the good advise listed above and change filters and check the clip on the trottle linkage to make sure it has not slid out of it's socket. This will have an intermittent impact as well dependent upon where the linkage has vibrated to.

Good Luck!