I have a necchi 4575 and when I sew a straight stitch it will sew just fine and then for a few stictches it wil act as if its out of thread but then it will pick uop and sew normal for a few more stiches anf then repeat the skipping any suggestions?

Firstly, the obvious stuff, is the needle blunt? This will caused skipped stitches; a needle is good for about 4-6 sewing hours or one project so needs to be changed regularly.

However, if you have already checked that; then it is possibly a case of the timing being slightly out on the needle bar vs rotary hook rotation.

If you look at this link, you'll see images relating to hook timing

http://www.sewusa.com/Sewing_Machine_Repair/Sewing%20Machine%20Hook%20Timing.htm

It is something that can be tackled yourself if you want to have a go. But if the machine hasn't been serviced for a while, then its probably worth taking it in for a professional service and timing as the tech can check all the settings and clean and lube internally.