You wont believe this. I finally got it to work.

Everytime I ran my Service the printer job got stuck in the queue with status "spooling...". I discovered that the following code segment somehow upheld the printing job.

while (WordApp.BackgroundPrintingStatus > 0) { sleeeeeeeeep for 250 milliseconds }

Removing this solved the problem!! The printerjob never get stuck, and runs smothly.

Unbelievable..