  We explore the region of small sea quark masses below $m_{PS}/m_V=0.5$ in
two-flavor QCD using a mean-field improved clover quark action and an
RG-improved gauge action at $a \simeq 0.2$ fm on $12^3 \times 24$ and $16^3
\times 24$ lattices. We find that instability of the standard BiCGStab
algorithm at small quark masses can be mostly removed by the BiCGStab(DS-$L$)
algorithm, which employs $L$-th minimal residual polynomials with a dynamical
selection of $L$. We also find singular spikes of $\Delta H$ in the HMC
algorithm at moderate values of $\Delta\tau$. Nature of the spike is studied.
We also study finite-size effects and chiral properties of meson masses.
