Yep, carbon paste and a slice of beer can. Or soda, whichever you have around. Usually about 1-2 cm wide and a few more long.

EDIT: Lots of carbon paste. I had the same year, same problem. Do not over-tighten the mounting bolts or new frame for you.

What Fishgo said about the carbon paste.

When I had a specialized transition I had this issue. Put a thin layer of carbon paste on it, only slid down half as much. Lathered the carbon paste on it and inside the seat post area never thought about it again. I've shimmed posts in the past as well no issues.

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Brian Stover USAT LII
Accelerate3 Coaching
Insta

i struggled with a slipping post on my 3T. I tried a thin layer of carbon paste and that only helped a little. What i would have considered "way too much" paste ended up being "just enough" apparently. It's been good for months this time. FWIW, i used the Finish Line brand this time.

Some of the 3t posts are notorious for sliding. Someone posted the idea of using a bit of the adhesive sandpaper on the inside of the aluminum piece that pushes up against the seatpost. So I guess this is sort of a combo shim and grit solution.

Spoke to a guy on one of their sponsored teams: Said that he had to JB weld the Strada seatpost to get it to stick. The company was no help.

Same as everyone else....although my TT bike is an aluminum P3-SL. I have shimmed my seatpost using a thin piece of brass. I bought several thicknesses from the local hobby store and made shims from each. It took a few days of experimentation to find one just the right thickness and size to stop my post from sliding.

The shim I made was about 3cm square. I folded it in half---to fit around the trailing edge of the seatpost. Then I cut down the crease by about 3mm, and folded those edges over square to create small lips on each side. These catch on the edge of the seat tube to keep the shim from falling down into the frame as you loosen and move the seat post.

I have had this problem on my speed concept. I used paste plus a narrow piece of old inner tube double over. It has worked great. Bike shop in Rothenburg ob der Tauber helped me with this between Challenges Roth and Regensburg in 2016. It has worked great, I just have a little flap hanging out the back of the seat tube.

Why not tightening it Down harder rather than shimming? - effectively it does the same thing, making less room. Not overly tight, try 5nm instead of 4nm and see if that works. 4nm seems very light for a seatpost

Because that's how you break things. Increasing the clamping force by 25% seems like it might not be a good idea

But, if you shim it and still tighten to 4nm, it’s as loose as before, only the isn’t threaded in as far

Going to give you my two cents here. Between myself, my two kids and my wife we have owned for Four 2008 through 2011 P3 frames. We still have 3 of them which we ride frequently and we still race on them. I never, ever once used a torque wrench to tighten the seatpost bolts, I just used common sense when getting the bolts tight. We never had a seat post slip and I have probably done HUNDREDS of seatpost changes on these bikes because my kids were constantly growing and I am always fiddling with my seat post height. I’ve gotten these bolts very tight and never had a problem with any type of damage. 4n/m is very low torque, I suggest simply tightening the bolts a little bit more, they will be fine.

Bingo!

Until you see how many frames come into the workshop with inserts pulled from the frame and the carbon cracked around them.
Going to 5nm is probably OK, but it is hard to set some cheaper wrenches and most have a 4% variance on top and the carbon can suffer from creep under too high tension and die over time, so a bit of a minefield.
Torque values vary so much because of varying bolt/thread lubrication that it is a bit of a crap shoot anyways, so going over is a bit iffy.

Often the do up to sensible IS the correct solution, but you do need to have mechanical sympathy and a feel for when things start to ramp, most people don't.

The correct solution in this case is choice of carbon paste.
There are varying coarseness pastes and choosing a gritty one will be the cure in this situation.

Thank God I have the feel.

I had the same issue with my P2C, drove me nuts. Nothing worked. Then took it to a good bike shop and they ground the seatpost clamp down on the edges by about a mm. (The flat areas that will push against the frame). Remember this clam has a v-shaped cutout and this V is that is pushed against the seatpost holding it in place, but if the clamp itself is a little too big it doesn't matter how hard you torque the flat areas of the clamp against the frame the V will always have too much room and not press hard against the seatpost.

So grind down the edges, leave the V itself untouched and it should work. Worked for me for 7-8 years perfectly since then.