underdriving pullys too small?
 

im currently running a pettit underdrive main pully along with underdrive water pump and alt pullys. ive notice that if i cruise at under 1800 rpms my greddy water temp (sensor behind uim) will drop off, leading me to believe coolant is not cirulating through the system as it should. if i blip the throttle or downshift the temp will jump for around 70 up to 80 some....additonally at low rpms i notice the voltage drop from about 13.6 to 12.4 since the alternator isnt spinning fast enough.....

could this potentially be a bad thing? as long as i keep cruise around 2500-2900 i notice no issues. what performance gain am i seeing from running this pully setup?