I have a 2000 ford flexfule and replaced the fuel pump and the gage says it's full all time?

When it gets freezing cold out it looks like it works for a short time then go's back to full r

Its unplugged or has a bad connection. The sender can "snag" as it passes through the hole in the top of the tank. Some ford senders are plastic snapped onto the steel. This can simply come unfastened by accident. Even when using a complete 9h307 pump and sender assembly. If you attached a new pump to the previous part there are several ways to have it disconnected, and in such case there's a possibility of an open in the reused sender. At any rate you must retrace your steps and verify ohms to ground can be read on the sender circuit before finally reinstalling

the tank IS ground. So you have to take it back out

Easy things first - try cleaning the ground connection for the fuel pump and for the sender, if it is not the same. I hope they are not really hard to get to. I have seen them become intermittent in other cars because they were screwed down to painted metal.