Read all these posts again.
Fred's looks good because it has a good chance of pointing at, or eliminating, the gas supply to the carb which is the subject of many of the other posts..
A plugged gas tank cap might cause a problem but as soon as you take the cap off and replace it, it will run a lot longer before it dies the first time. This makes this problem unlikely but Fred's test will show it.
Fred's test will also show a problem with the float and needle valve.
You may still have an ignition problem. If your ignition is electronic, it could be temparature sensitive, if points, you could have a bad capacitor.
Do Freds tes and if it points you to the problem, you are good to go. If it does not, check spark and repost.