I try to solve an integro-differential equation with a convolution term using this code. Here i used a periodic condition.

    L=10; tmax = 2;

    NDSolve[{D[u[x, t], t] + u[x, t]*D[u[x, t], x] + D[u[x, t], {x, 2}] + 
    D[u[x, t], {x, 4}] + 1/(2 L)*D[NIntegrate[u[xp, t]*Cot[\[Pi](x - xp)/(2*L)], {xp, -L, L}(*,PrincipalValue\[Rule]True*)], {x, 3}] == 0, u[-L, t] == u[L, t],
    u[x, 0] == Cos[\[Pi]/L*x]}, u, {x, -L, L}, {t, 0, tmax}]
which gives me
>NIntegrate::inumr: "The integrand ... has evaluated to non-numerical values for all sampling points"

The warning is understandable because the function `u[xp, t]` is still unknow when `NIntegrate` is evaluated. I also tried `Method -> "MethodOfLines"`, but it still doesn't work.

My problem may closely relate to [this][1], which is linear equation and more like a wave equation in the sense of $u_{tt}$. But my pde is nonlinear and has 1st-derivative in $t$. So I post my problem here for a more general solution. Thank you for any suggestion.

**Update:**

As indicated by bbgodfrey in a comment: it is necessary to convert the convolution to the same range as used in `NDSolve`. For a spatial periodic solution, I have changed the convolution with integral limits of infinite to a `NIntegrate` within a finite interval. However, with `Convolve` one can use `PrincipalValue -> True` for a principal value integral, with `NIntegrate` how could we calculate the Principal Value?


  [1]: https://mathematica.stackexchange.com/questions/107538/solving-pde-involving-hilbert-transform-numerically