I had 120 on 1 leg and 120 on other leg on a panel I had 120 and 75 volts on bottem of breaker I changed panel box ,it worked great for about 3 weeks now its doing same thing what would be causing this thanks

If you have 120 Volts at top of panel, you should have at the bottom of panel, unless you are measuring from a 2 pole breaker with a burnt breaker contact or bad pole of 2 pole.
In this case you may be reading 1 leg through load and returning back to breaker. Measuring Hot to hot from breaker will determine if back feeding.

Yes I understand that what would cause this twice

I can't follow. One BIG run-on sentence. If it's the main lug, this could be a poor connection anywhere from the transformer to the box. It's a utility issue.

Bet if it's at the main lug and you turn off the main CB, the voltages measure normal again.

I can't follow. One BIG run-on sentence. I agree. Maybe a better explanation of what is going on will help us help out.

Where specifically are these reading being taken?

Its on the main disconnect .100 amp panel

yes i understand that what would cause this twice
A Same Bad breaker at botom of panel, are you measuring off breaker?
If you had burnt buss and breakers before, and change only the panel and not burnt breakers, The breaker needs to be changed or it can not only fail again, but do damage to panel.

If you are measuring off breaker, also check voltage from hot to hot, Is this a 2 pole beaker?

Its on the main disconnect .100 amp panelI'm not sure if this is "specific" enough. :rolleyes:

I mean like what terminals, what points are you measuring from?