What you are experiencing is called drift and is perfectly normal. Get you PH to 5.5 and then watch it climb up to around 6.1 before you readjust. Just adding new RO water should help you stabilize doing top offs somewhat. With Lucas though you can also top off with 1/3rd strength nutes, you'll just want to keep an eye on your PPM's and back off if they rise too much.
A PPM of 1000 is way two high for a two week old plant. If you are going to go with the Lucas formula and you plan to put plants in to flower after two weeks you will want to start out in the 400 range and adjust week by week only adding a small amount until you reach a PPM of about 1300. This however is based on using RO water.
What temp are your nutes being distributed at? It's also very important to keep those temps in check.