I'd suspect an issue with the kit first. But you'd need another board to test it in (or another kit). I'd also just remove the kit and make sure you don't see the same behavior in stock test mode (which would suggest a game board issue). I'm not aware of any obvious issues (that I've seen anyway) that would cause that behavior.
Other things:
Make sure you have good 5V on the game board. Remember that Asteroids AR's read artificially high when measured on the AR with no game board plugged in. You should get about 6.2V on the AR's 5V test point with no load, which will come down to around 5V on the game board when it's plugged in. Ultimately you want to measure on the game board and set the AR pot so it's 5.00V, using the game board's 5V and GND test points.
Make sure you don't have any CP buttons (or coin switches, or the slam switch) stuck on. You could try just unplugging the whole CP, as the test switch will still work on the coin door.
Also, if this is a Braze kit, make sure you have an LS08 on the smaller kit daughterboard, not an ALS08. See this thread:
SOLVED: Braze Multi-Game random vectors, lunar lander only... I installed the Braze Multigame-Kit into my Asteroids this weekend. Install was super simple, and everything works as it should EXCEPT when I'm playing Lunar Lander, i get random vectors that pop up about every 5 seconds. They appear...
forums.arcade-museum.com
Also, you said in the OP, "so I can use the LL thruster". How are you doing that? Do you have douglasgb's A/D interface installed?
The ADL (Asteroids/Asteroids Deluxe/Lunar Lander) A2D (analog to digital) Thruster Interface is a small circuit board designed to plug into…
highscoresave.com
If so, that could be another variable. I'm not really familiar with it, but you could try removing it, and seeing if that makes any difference.