FLC OF ACR MLG ACFT RESPONDED TO TCASII TA RESULTING IN DEVIATING FROM ASSIGNED ALT.

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AIRBORNE AT PM13 CDT. UNDER DEP CTL LEVEL AT 5000 FT; 250 KTS; HDG 090 DEGS. TCASII WAS OPERATIONAL IN TA/RA WITH 10 MI RANGE SELECTED AND SEVERAL NOT THREAT TARGETS DISPLAYED WITH ALT. FLT CONDITIONS WERE IMC WITH VERY LITTLE VISIBILITY. YELLOW (TA) DOT SUDDENLY 'POPPED' UP ON TCASII SCREEN WITH AURAL WARNING. IT WAS DIRECTLY AHEAD OF US AND INSIDE 2 1/2 MI RANGE MARKER. DOT HAD NO ALT WHICH IS WHY I BELIEVE WE DID NOT GET AN RA. WE WERE CLOSING RAPIDLY AND I STARTED AN EVASIVE R TURN AND DSCNT. THE CAPT ADVISED ATC THAT WE HAD A TCASII ALERT AND WERE TURNING R AND DSNDING. WE CLRED CLOUDS AT ABOUT 4000 FT HDG ABOUT 180 DEGS. TA POS HAD STAYED ON OUR NOSE DURING INITIAL PART OF TURN. NOW IN CLR; WE SAW NO TFC AS ATC SAID THEY HAD AN MLT (OFF STEWART; I PRESUME) AT OUR 9-10 O'CLOCK. WE CLBED BACK TO 5000 FT AND TURNED TO 090 DEG HDG WITH ATC CONCURRENCE. I DON'T BELIEVE WE EVER RECEIVED ALT READOUT ON TA. OUR OPS MANUAL SAYS NOT TO RESPOND TO TA; ONLY FOLLOW RA. HOWEVER; IT ALSO STATES TO VISUALLY SCAN (CLR) THE AIRSPACE YOU INTEND TO MANEUVER INTO. YOU CAN'T DO THIS IN THE CLOUDS. THE TA SITUATION IN CLOUDS IS NOT COVERED. I FIND IT DIFFICULT; IF NOT IMPOSSIBLE; TO WATCH THAT YELLOW DOT CONVERGE WITH OUR POS AND NOT TAKE ACTION.