"Why was Rumjantsev (1st TA & 5th GTA) stopped dead in their tracks in August 1943 at Bogodukhov?"
I'm not sure which action this was, but i believe they were stopped by 48 panzer corps, which was heavily mechanised and motorized. Anyway, after taking heavy losses, 5th Guards marched 200 miles north (i think it was north, could be wrong) and attacked where the German divisions weren't so tough (or mobile).
I think 1943 was a very pivotal year for the East Front. That was when Lend-Lease really began to take effect via the famous Murmansk and not so famous Vladivostock route. Perhaps i would not be amiss in saying that before Kursk many German divisions had at least some mobility, but that after Kursk Russian production and lend-lease trucks and AFV allowed the Russians to be far more mobile than the Germans, especially after the Kursk losses.
Right near the end of 1943 and certainly into 1944 the Germans began producing massive numbers of AFV, enough to almost catch up to the Russians. However, i'd like to point out that most of those AFVs went to the West Front, not the East.