using an alert just after
var q7_sum = q7_1Val+q7_2Val+q7_3Val+q7_4Val+q7_5Val+q7_6Val+q7_7Val+q7_8Val+q7_9Val+...
line should hint on the cause of the problem
you will probably get something like "102000050", I guess the variables are initialized as string. So either change your declarations to var q7_1Val = 0 and then assign with SSI_GetValue or use parseInt(SSI_GetValue()) to make sure they are handled as numbers.
Another thing might be if you are using a constructed list for the constant sum, if that is the case you need to check existence of each Q7_x so that it doesnt cause JS errors