1) On a old laptop i had it had the same problem were the fps will drop to 1 after 5mins of game play and it lagged for about a 1-2mins every so often, this was due to the cpu getting too hot (95C) so it shut down one core of the two to cool down which caused the lag. If this is your problem reply with you cpu usage in windows task manager when it happens.
2) On my new laptop the GPU is not used by default which cause lag once the world has loaded. If this is your problem? right click on technic launcher and hover the mouse over "Run with graphics processor" and select the correct graphics card.
Please post your computer specs for more help.