BuildCraft version: 7.99
Forge version: 14.23.4.2705
Link to crash report or log: N/A
Singleplayer or multiplayer: Singleplayer
Steps to reproduce: Random
Additional information: N/A

This happened to me too. When I opened a stirling generator's GUI, the game froze for about 20 secs and then all is back to normal. Opening that GUI again doesn't freeze game.
Mod list if needed:
jei_1.12.2-4.11.0.202
gregtech-1.12.2-0.4.5.9
CodeChickenLib-1.12.2-3.2.0.345-universal
industrialcraft-2-2.8.91-ex112
buildcraft-7.99.17
forge1.12.2-14.23.4.2705

can you guys provide a log file from when this happens? or even a thread dump if you can get one, that might show what's going on here