Levenberg–Marquardt not converging - c++

Question

I try to make a model fit using Levenberg-marquardt's method according to numerical recipes.
The Problem is: it does not converge or when it does, it's not precise... or at least the covariant matrix is strange.
int i=0;
for (i = 0; i < 3e4; i++) {
mrqmin(x, y, sig, NPCalib, a, ia, 3, covar, alpha, &chisk, afunc,
&alamda);
if (chisk < 1e-8)
sumchisk++;
if (sumchisk > 5)
break;
if (alamda > 1e8)
alamda = 1e8;
}
(x,y) are 3 points (double) that work pretty well with the form y=a(x-x0)^2.
using sumchisk like this is the recommendation of numerical recipees for using this function.
alamda is capped at the top here as otherwise there might have been an overflow.
Other definitions and data-points:
double a[4] = {0.0, 0.0001, 100.0, -1};
int ia[4] = {0.0, 1, 1, 0};
double *x = {0.0, 799.157549545577, 799.92196995454, 800.683769692575};
double *y = {0.0, 524.26491, 525.26768, 526.26586};
double *sig = {0.0, 0.1*y[1], 0.1*y[2], 0.1*y[3]};
double **covar = new double*[4];
covar[1] = new double[4];
covar[2] = new double[4];
covar[3] = new double[4];
double **alpha = new double*[4];
alpha[1] = new double[4];
alpha[2] = new double[4];
alpha[3] = new double[4];
double chisk = 0;
double alamda = -1;
void afunc(int i, double x[], double a[], double *y, double dyda[], int ma)
{
*y = a[1] * pow(x[i] + a[2], 2) / pow(1 + a[3] * CT[i - 1], 2);
dyda[1] = pow(x[i] + a[2], 2) / pow(1 + a[3] * CT[i - 1], 2);
dyda[2] = (2 * a[1] * (x[i] + a[2])) / pow
(1 + a[3] * CalibTurn[i - 1], 2);
dyda[3] = (-2 * a[1] * CT[i - 1] * pow(x[i] + a[2], 2)) / pow
(1 + a[3] * CT[i - 1], 3);
}
I changed the nr-sourcecode to use double instead of float. The first array-element is not used because this comes from fortran-code and I didn't feel like changing such a small detail.
The model also contains a 3. parameter, which isn't used in this fit and thus remains a[3]=-1, because ia[3]=0. ia[]=1 means the parameter is about to get fitted...
However, Now I have the problem that sometimes this doesn't converge. It finishes with alamda=1e8 and i=3e4. Especially when I set the treshold for chisk lower.
The sets of parameters seem to be fine, though... the chisk is e.g. about 1e-6 and the parameters seem fine, but looking at the diagonals of the covariant-matrix (which should give the squared standard deviation of each parameter), there is some rubish like ~800000 for a parameter 0.0001.
Does anyone know what I did wrong when using this algorithm?
Anything specific I need to write into covar/alpha when I start? Can the sig be set like this?