First problem is you are using the wrong lens 50mm for 6x6 negatives. Put a 80mm lens in the enlarger and some or most of your difficulties will be eleminated.
I also recommend a glass carrier, reg glass on bottom and anti newton on top for printing. This will hold the negative flat in the carrier and stop the middle of the negative drawing to the light source in longer exposures.
If these two things do not solve your problem then I would check the enlarger alignment and such.