The 1981 ABC Guide on Timetable World should be able to help you.
In 1986 the weekday service ran about hourly from Peterborough to Hitchin stopping at all stations 02h50, 05h30, 06h27*, 06h47*,08h36*, 10h08, 10h54, 12h05, 12h53, 14h01, 14h53, 17h29, 21h02 [*fast through to KX], with short workings from Huntingdon to Hitchin stopping at all stations at 07h53 (through to Hertford), 09h13, 16h09, 16h45, 18h25, 19h52, 22h32. Also shown marked '125' is 07h55 Huntingdon to King's Cross; all stations to Biggleswade then King's Cross.Indeed, have been looking at that recently.
In 1981 it appeared to be a regular, clockface hourly service from Hitchin to Huntingdon calling at all stations off peak. (In the peak some through Kings Cross to Peterborough services ran, presumably loco-hauled).
It was timed to connect at Hitchin with a Kings Cross-Royston limited stop.
That said, I am fairly sure that a bit later (1984/5) the DMU shuttle became less clockface and less regular, though don't quote me on that. Maybe they took it down to a single DMU. (I did have plans to visit somewhere like Stevenage around that time, to get a taste of the ECML - and got as far as examining the timetables in the area - though it never actually happened)