Suppose that $f$ is a weight $k$ newform for $\Gamma\_1(N)$ with attached $p$-adic Galois representation $\rho\_f$. Denote by $\rho\_{f,p}$ the restriction of $\rho\_f$ to a decomposition group at $p$. When is $\rho\_{f,p}$ semistable (as a representation of 
$\mathrm{Gal}(\overline{\mathbf{Q}}\_p/\mathbf{Q}\_p)$?


To make things really concrete, I'm happy to assume that $k=2$ and that the $q$-expansion of $f$ lies in $\mathbf{Z}[[q]]$. 


Certainly if $N$ is prime to $p$ then $\rho\_{f,p}$ is in fact crystalline, while
if $p$ divides $N$ exactly once then $\rho\_{f,p}$ is semistable (just thinking about the Shimura construction in weight 2 here, and the corresponding reduction properties of $X\_1(N)$
over $\mathbf{Q}$ at $p$). For $N$ divisible by higher powers of $p$, we know that these representations are de Rham, hence potentially semistable. Can we say more? For example,
are there conditions on "numerical data" attached to $f$ (e.g. slope, $p$-adic valuation of $N$, etc.) which guarantee semistability or crystallinity over a specific
extension? Can we bound the degree and ramification of 
the minimal extension over which $\rho\_{f,p}$ becomes semistable in terms of numerical
data attached to $f$? Can it happen that $N$ is highly divisible by $p$ and yet $\rho\_{f,p}$ is semistable over $\mathbf{Q}\_p$?


I feel like there is probably a local-Langlands way of thinking about/ rephrasing this question, which may be of use... 


As a possible example of the sort of thing I have in mind: if $N$ is divisible by $p$ and $f$ is ordinary at $p$ then $\rho\_{f,p}$ becomes semistable over an abelian extension of
$\mathbf{Q}p$
and even becomes crystalline over such an extension provided that the Hecke eigenvalues
of $f$ for the action of $\mu\_{p-1}\subseteq (\mathbf{Z}/N\mathbf{Z})^{\times}$ via the diamond operators
are not all 1.



Galois representations attached to newforms
The right way to do this sort of question is to apply Saito's local-global theorem, which says that the (semisimplification of the) Weil-Deligne representation built from $D\_{pst}(\rho\_{f,p})$ by forgetting the filtration is precisely the one attached to $\pi\_p$, the representation of $GL\_2(\mathbf{Q}\_p)$ attached to the form via local Langlands. Your suggestions about the $p$-adic valuation of $N$ and so on are rather "coarse" invariants---$\pi\_p$ tells you everything and is the invariant you really need to study.


So now you can just list everything that's going on. If $\pi\_p$ is principal series, then $\rho$ will become crystalline after an abelian extension---the one killing the ramification of the characters involved in the principal series. If $\pi\_p$ is a twist of Steinberg by a character, $\rho\_{f,p}$ will become semistable non-crystalline after you've made an abelian extension making the character unramified. And if $\pi\_p$ is supercuspidal, $\rho\_{f,p}$ will become crystalline after a finite non-trivial extension that could be either abelian or non-abelian, and figuring out which is a question about $\pi\_p$ (it will be a base change from a quadratic extension if $p>2$ and you have to bash out the possibilities).


Seems to me then that semistable $\rho$s will show up precisely when $\pi\_p$ is either unramified principal series or Steinberg, so the answer to your question is (if I've got everything right) that $\rho\_{f,p}$ will be semistable iff either $N$ (the level of the newform) is prime to $p$, or $p$ divides $N$ exactly once and the component at $p$ of the character of $f$ is trivial. Any other observations you need should also be readable from this sort of data in the same way.


One consequence of this I guess is that $\rho\_{f,p}$ is semi-stable iff the $\ell$-adic representation attached to $f$ is semistable at $p$.


