An EMB-145 Captain described experiencing Air Data Computer failure in flight resulting in Airspeed and Altitude Mis-Compare EICAS alerts. He elected to return to departure airport for maintenance.

Narrative

While climbing through FL300 we encountered an airspeed mis-compare error; followed shortly by an altitude mis-compare. We assessed which instrument set was accurate by using the stand-by instruments. Since the First Officer's instrumentation was indicating correctly; I transferred control of the aircraft to the First Officer and began the QRH. I then contacted Dispatch and informed them of the situation and was then handed over to Maintenance Control. We then began explaining what indications we were getting; and while we were discussing this; the errors went away for about 30 seconds. Maintenance Control informed me that if there are no present errors; we are okay with you continuing on to your destination. At that time the airspeed error came back and I then told Maintenance Control I was returning to departure airport and needed to talk to Dispatch; we then did an ADC reversion and returned which had significantly better weather than destination. Maintenance had changed out AHRS#1 in response to an earlier write up mirroring what we had experienced; so this was not a first time problem this day. With the error messages presented; which were: IAS Mis-Compare; and ALT Mis-Compare on both Primary Flight Displays; SPS Advance; and AHRS 1 Basic Mode on EICAS; and ADC1 fail on the FMC; that combined with the weather at destination (forecast at our time of arrival to be winds out of south west at 20 gusting 35; overcast 500 FT heavy rain; visibility 1 SM strong thunderstorms) I felt that the safer course of action was to return where we had a relatively thin 2000 foot thick layer with bases around 4000 FT and clear skies directly overhead.