COMMUTER ACR LTT CLRED RWY HEADING BEHIND DC9 CLB 4000 FT. HELD 140 KTS IN CLB FOR WAKE TURB AND HAD CLB RATE OF 3000 FPM. NEARING 4000 FT VECTORED TO 110 DEGS RESULTING ALT OVERSHOOT OF 300 FT.

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FLT DEPARTED RWY 9 BEHIND A DEPARTING MD90 AND WAS GIVEN RWY HEADING. 140 KIAS WAS HELD FOR WAKE TURB AVOIDANCE RESULTING IN A 3000+ FPM RATE OF CLB. CLRNC WAS TO CLB AND MAINTAIN 4000 FT; WE WERE HANDED OFF TO DEP AROUND 3500 FT TO 3700 FT AND WERE INSTRUCTED TO TURN TO HDG 110 DEGS. THROUGH 4000 FT WE RAPIDLY DROPPED THE NOSE BUT WERE UNABLE TO ARREST THE CLB TILL 4300 FT AT WHICH POINT WE RETURNED TO OUR ASSIGNED ALT. THE LEVELOFF AND DSCNT TO 4000 FT LASTED LESS THAN 5 SECONDS AND WAS BEING CORRECTED AT ALL TIMES. THIS EVENT COULD HAVE BEEN AVOIDED BY ATC GIVING US A SLIGHT TURN UPWIND OF THE DEPARTING MD90 SO THAT A NORMAL CLB COULD BE ACCOMPLISHED AT A LOWER RATE OF CLB. ALSO; THE LATE HDOF INTERRUPTED THE CAPT'S '500 FT TO GO' CALL; SO THAT HE WAS UNABLE TO MENTION ALT UNTIL I WAS ALREADY PASSING 4000 FT. I ALSO BELIEVE 2 CONTRIBUTING FACTORS WERE THAT I WAS FLYING WITH AN UNFAMILIAR CAPT WHOM I HAD ONLY FLOWN WITH ONCE BEFORE SEVERAL MONTHS EARLIER; WHICH MAY HAVE MADE ME A LITTLE UNCOMFORTABLE OR NERVOUS; AND WE HAD AN FAA INSPECTOR RIDING ALONG WHO SHOWED UP AFTER I WAS ALREADY IN THE ACFT PREPARING FOR BOARDING. I WAS A LITTLE SURPRISED TO SEE HIM BECAUSE USUALLY THEY CHK IN EARLY AND WE ARE INFORMED BY DISPATCH. THE SUDDEN CHK RIDE SCENARIO MAY HAVE ALSO MADE ME NERVOUS AND SLIGHTLY DISTRACTED MY ATTN MOMENTARILY TO THE LEVELOFF.