overflowcount++;
How long do you think that takes?
Look at the asm code.

Plus it is not atomic. You need a semiphore (flag) to control access to it.

without comments or formating you code is hard to understand.
Add comments and use the pre /pre tags.

I can only guess what you are trying to do. If you are trying to use the interupt to generate a delay that is not the way to do it. Tell us what you want to do, not what you are doing.

That giant switch statement could be replaced with 1 or 2 lines of code with math.

this should be a while statment, to start with.
lv:
if(overflowcount!=1800)
goto lv;

The LCD up dates slowly plus you have no filtering. Fast update should be avoided.