88 190E 2.3

Hi,

I was wondering if somebody could assist me.  The camshaft bearing bolts (16 total) on my 1988 190E dont seem to hold the torque. A few days ago I replaced the camshaft. I torqued down each set ( 4 sets) of camshaft bearing bolts  to 15 ft-lbs as stated in Haynes manual. After torquing down all 4 sets of camshaft bearings I notice that the previous ones needed to be torqued further. I closed the valve cover and drove the car for a few days. It runs beautifully smooth. Not satisfied I reopened the valve cover and found that some of the bolts have loosened and need to be torqued down again? Some questions:

1. Is 15 ft-lbs the correct torque?

2.  Is there another proper procedure to torque down these bolts?

3. How do I ensure that the bolts hold the torque as specified?

Any assistance would be much appreciated.

Shakil