IFR ENRTE M7 DEPARTS ASSIGNED ALT IN ZFW AIRSPACE.

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ACFT X AN M7 FROM WILEY POST TO PARIS; TX; WAS AT 5000 FT MSL APPROX 3 NM NE BYP WHEN I ADVISED HIM 'RADAR CONTACT LOST.' ABOUT 5 MINS LATER I RE-IDENTED HIM. VERY SOON AFTERWARD I LOST HIM AGAIN; AND AGAIN INSTRUCTED 'RADAR CONTACT LOST.' AT ABOUT XA44 I SEE A TARGET NW OF PARIS SQUAWKING XXXX (SAME AS THAT ASSIGNED TO ACFT X) WITH A MODE C READOUT OF 4300 FT. I ASKED THE PLT TO VERIFY LEVEL 5000 FT. HE RESPONDS 'WE'RE LEAVING 4100 FT.' I ASKED HIM IF HAD RECEIVED A CLRNC FOR LOWER; AND IF HE WAS STILL ON AN IFR FLT PLAN. THE ANSWERS WERE (PARAPHRASED) NO AND YES; RESPECTIVELY. I THEN ASKED HIM TO 'SAY INTENTIONS;' TO WHICH HE INDICATED HE WOULD CANCEL IFR. RADIO COVERAGE IS LIMITED IN THIS AREA; BUT PROBABLY NOT A FACTOR; AS COM WAS INTACT THROUGH 4000 FT. IF THE PLT WAS UNABLE TO TALK TO CTR TO REQUEST LOWER/CANCELLATION; THERE WERE OTHER ACFT ON FREQ THROUGH WHICH TO RELAY. THERE WAS ANOTHER ACFT APCHING PRX FROM THE E; NON RADAR; AND ACFT MAY HAVE BEEN EXPECTING LOWER; BUT DID NOT RECEIVE THIS CLRNC DUE TO THIS TFC.