Hi-- I've been following your delema. Sorry but if I'm thinking right 5 pounds would be 80 ounces not 60. You can figure at 700 sq. in. divided by 144 sq. in. (1 sq. ft.) you have 4.8 sq. feet of wing area and if you divide 4.8 sq. ft. into 80 oz. you have a wing loading of 16.6 oz. per sq. ft. That is a lot of loading -- way to much for your engine. Have you thought about dropping the engine and using a wench or heavy duty hi-start? When you said you had a few thud landings (if they were flat thuds--not nose sticking in) that will tell you that you were not even close to the air speed it will need. I if it were me I would check the wing to stab incedence and the CG to make sure there right and make sure everything is square and straight. I would also take it out to a grassy field with about a 10 mph wind (no engine running) and throw it level at the horizon and find how hard I needed to throw it to get at least a 20 yard glide with a smooth landing. With a 10 mph wind, throwing straight into the wind, you will already have 10 mph air speed before you even throw it. Oh make sure the CG is a little nose heavy - you don't want to stall it that low. Well that's what I'd do before trying a hill or high up throw - there are usually turbulances coming up those places.
Good Luck - Hope this helped [8D]