I'm working on LAMP for the past few months. I have few hints to solve your false positive problems.
1. NEVER use same pippetor to make master mix and to load LAMP product in agarose gel.
2. Check the position of your primers in the target gene. If your F3 and F2 primers are side by side or too near, it might results false positive. Make sure there is some distance between F3 and F2 primers. Check out some journals for the illustration of the primers position.
3. If you designed more than 1 set of primers, you can mix match your primers as suggested by someone in this thread to avoid false positive. When mix matching make sure there is some distance between each primer.