Here are some things I would try.

1) Change fuel tubing (sounds like you already did this)
2) Make sure you have good, fresh fuel
3) Make sure you have the tank positioned correctly (the lines coming out of the tank should be horizontal to the engine carb)
4) Make sure you don't have any air leaks (good seal on tank, good seal under carb, good seal at glo plug, good seal on backplate, etc.)
5) Make sure you have your engine running about 300 RPM's rich or peak

Now for the $75 "sure thing" fix. Buy a Thunder Tiger 46Pro and watch all your problems go away. The OS LA series is an underachiever. Just look through the threads in this forum. The TTPro is the best value in R/C 2-stroke engines - period.

Good luck