AN MD80 DEPARTING AFTER HVY RAIN EXPERIENCED UNEXPLAINABLE HDG AND AIRSPD FLUCTUATIONS LASTING APPROX 30 SECONDS AND CAUSING A 500 FT ALT OVERSHOOT.

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DURING CLBOUT FROM LGA ON A CONEY CLB; BOTH COMMAND BARS WOULD APPEAR AND DISAPPEAR AND THEN GIVE UNRELIABLE GUIDANCE. WE WERE ENCOUNTERING MODERATE TURB AND TRYING TO INTERCEPT THE 043 DEG RADIAL FOR THE DEP. BOTH HDGS BECAME UNRELIABLE SHOWING APPROX 15-20 DEGS DIFFERENCE. AS THE HDG BARS WOULD REAPPEAR; THE FLT GUIDANCE COMPUTER WOULD DEFAULT INTO HDG HOLD WHICH WOULD REQUIRE US TO RESELECT HDG SELECT. THEN THE COMMAND BARS WOULD DISAPPEAR AND THE SEQUENCE WOULD BEGIN AGAIN. DURING THIS PROCESS; WE CLBED TO APPROX 5500 FT BRIEFLY BEFORE DSNDING BACK TO 5000 FT. OUR HDG FLUCTUATED +/-20 DEGS AS WE TRIED TO DETERMINE THE CORRECT HDG. ALSO; THE AIRSPD INCREASED AT ONE POINT TO 280 KTS. WE WERE GETTING BOUNCED AROUND QUITE A BIT AND TRYING TO FIGURE OUT WHAT WAS WRONG WITH THE ATTITUDE AND HDG SYS. FINALLY; AS WE CLBED THROUGH ABOUT 10000 FT; THE SYS APPEARED TO WORK NORMAL. AS WE CONTINUED THE CLB; THE SYS SETTLED DOWN AND WORKED RELIABLY THE REMAINDER OF THE FLT. THE CAPT DID WRITE UP THE PROB IN THE LOGBOOK. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR (FO) STATED THAT THE ENTIRE EVENT LASTED ABOUT 30 SECONDS BUT SEEMED MUCH LONGER. HE ATTEMPTED TO REMEMBER ALL OF THE INDICATIONS BUT THEY BECAME SO VARIED THAT HE WAS UNABLE TO PUT THE RAPID FLUCTUATIONS IN ANY PATTERN. HE HAS RECEIVED NO FEEDBACK AS TO ANY MAINT DISCOVERY. SUPPLEMENTAL INFO FROM ACN 681595: NO COMMENT FROM DEP REGARDING ALTDEV. ADVISED THEM WE WERE EXPERIENCING EQUIP DIFFICULTIES. ACFT SETTLED DOWN AND OPS NORMAL AFTER ABOUT 10 MINS. SUSPECT HVY RAIN DURING OVERNIGHT MAY HAVE CAUSED EQUIP PROBS.