what now????

After removing the EGR Valve Vauxhall Astra (Exhaust Gas recirculation)we found tht piston to be seized and really dirty so we cleaned the oriface on top of the maifold then applied 12v to the two pins under the caps to the top of the EGR still the piston wouldnt move so we soked it in brake cleaner and applied a few small taps to the top of the piston to shock it free,we then left it ten mins and applied 12v again and it worked yippee!we assembled it back together and surely this must have cured the problem but after a run i still get the same problem.anyone got any ideas as to what to do next cheers andy