ALT DEV ALT EXCURSION CREATES A POTENTIAL CONFLICT WITH OPPOSITE DIRECTION TFC LTSS.

Narrative

AT APPROX 100 MI E OF PUEBLO; CO; ON J64 WE REQUESTED AND WERE CLRED TO CLB FROM FL350 TO FL390. AFTER INITIATING CLB; ATC ASKED THAT WE BE LEVEL AT FL390 IN 6 MINS. IN ORDER TO LEVEL AT FL390 WITHIN THE 6 MINS THE INDICATED MACH DECREASED FROM NORMAL .830 TO APPROX .774. THE CAPT DETERMINED THAT OUR SPD WAS TOO SLOW AND INITIATED A GENTLE DSCNT WITH MAX CONTINUOUS THRUST STILL ENGAGED. AT APPROX FL387.5 I INDICATED THAT WE NEEDED TO LEVEL OFF AND THE CAPT LEVELED AT APPROX FL387. ATC REQUESTED A VERIFICATION OF OR ALT AND I INFORMED ATC THAT WE WERE AT FL387 AND WOULD BE ATTEMPTING TO CLB BACK TO FL390. BEFORE INITIATING A VERY SLOW CLB (APPROX 100-300 FPM CLB) THE ACFT HAD DSNDED TO APPROX FL385. DURING OUR CLB BACK TO FL390 ATC RPTED XING TFC AT FL370 AND THAT; AT XX30Z; WE HAD A POSSIBLE ALT DEV AND SHOULD CALL THE DEN CENTER MGR ON ARR AT DEST. THIS INCIDENT OCCURRED BECAUSE OF A LACK OF COM BTWN THE CAPT AND MYSELF AND BTWN OUR ACFT AND ATC. I ASSUMED; IN ERROR; THAT THE CAPT WOULD ONLY DSND 200-250 FT TO BUILD SPS. WE SHOULD HAVE DISCUSSED WHAT HE INTENDED TO DO AND DETERMINED THAT ATC CLRNC OR AN EMER DECLARATION WAS NECESSARY TO RETURN THE ACFT TO NORMAL SPD AND ALT. SUPPLEMENTAL INFO FROM ACN 191786. WE REACHED FL390 AND I NOTICED OUR SPD AT MACH .774 AND DECAYING (I HAD BEEN DISTR BY A FLT ATTENDANT ASKING ABOUT OUR LOCALE). I ANNOUNCED TO THE COPLT 'I HAVE THE ACFT' AND EASED THE NOSE OVER TO REGAIN AIRSPD. ATC CALLED AND NOTIFIED US OF AN ALT DEV AND THAT I SHOULD CONTACT DENVER CENTER BY PHONE.