1.I would have checked,first thing, communication problems on the host or checking for heavy cpu process on the host.
A comms problem would affect all uer/queries i would think. I guess they were looking for the execution plan answer.

Another answer might be that the query itself had been changed de to an upgrade/problem fix, and not regression tested.

3.To enforce the optimizer choosing FTS instead of index access you should insert the the "FULL" hint in the "select" statement,like in the following example:

select /*+ FULL */
f_name,l_name,address
from emp
where emp_no=1;
Not really an enforcement, more a suggestion. The hint just lowers the relative cost of full and index scans -- doesn't guarantee a FTS i believe