The good news Keith that is has not a lot to do with weight ...unless you stomach effects you guitar beyond the capability of the strap. If you are new to guitar finger tiredness, cramp especially with barre chords is very common. Having issues playing clearly over a long period of time is very common. You are still building up a very specific type of muscle memory and you body needs to get "game fit".

To master you Rock God stance while standing up you probably need to have a look at the mechanics of you fretting hand (Im assuming left ...but either way the issues are just the same). When you sit down, with the guitar on your knee, your wrist ( the angle of your forearm compared to the back of you hand will be minimal and you fingers wrap around to the frett board. When you play individual notes (especially on the G,Band E strings) your wrist will probablly flatten slightly more.

If you sling your guitar like Slash (all low hung) you may find that you have to bend you wrist to be able to fret notes correctly (Because you have to reach down, to the neck, then curve you wrist around)..

The possible solution to this is, at least for now, bring your guitar up higher. This may mean shortening the strap or moving the head of the guitar up towards your head rather than parallell with the ground. A combination of both can be good. Depending on how new you are, playing clear notes while comfortable, however, is much more important. So don't be frustrated it's very common. It all takes time but you will get there :)

PS Im no expert but thats my 2 cents worth.