thanks for your reply but unfortunately when it come loose it develops the typical "loose headset" play that I hate so much (you can feel it by pulling the front brake and moving the bike back and forth)...I forgot to mention that the steering tube is 1.5"..
If your headset is becoming loose that means that your stem is slipping on the steering tube. That would explain why the top cap is becoming loose, since the slight movement of the stem and spacers up and down the steering tube could be slowly pushing it up, and loosening it.
If I'm not mistaken your bike should have a carbon steerer and a carbon stem. I'd recommend taking the stem off, cleaning both stem and steering tube at the interface points (where they touch each other), applying carbon mounting paste to the area and putting the stem back. Of course, you should tighten it using a torque wrench.
Apply the mounting paste after you've dropped in the lower spacers, otherwise the spacers could push some of the mounting paste (which is abrasive by nature) down into the headset bearings, and you definitely don't want that.
With a correctly preloaded headset bearing and correctly torqued stem you could even get rid of the top cap and it wouldn't develop any wiggle or looseness. Some people actually do that to shed a few grams, but I don't recommend it.
Remember that the top cap has 2 functions:1.
It allows you to preset the headset bearing tension.2.
It prevents (more or less) water from entering your steerer and headset bearings.
Some people claim that the stem already seals that area from the top well enough, but when you're riding and pulling on the handlebars even the stiffest stem will deform and "rock" up and down / side to side very slightly, but that's enough for water and sweat to flow through that minuscule gap and ruin or corrode your headset bearings sooner than expected.
Water and sweat won't mix with the grease that's down there, but the repetitive movements will end up creating a dirty emulsion of grease + sweat + water that is not exactly healthy for those materials.
/end of rant.