Pogostemon growth

Pogostemon stellata is a very attractive aquariaum plant and looks fantastic in any tank where it is growing well and is vibrant. The discolourous foliage (green on top, pink beneath) is a great attraction when the leaves wave in the filter current.
I have few problems with this plant, but do regularly experience grow irregularities resulting in stunted leaves and short internodes. The plants always recover from these symptoms within 4-5 days and growth resumes either as a single apical shoot or as branched growth. Plants may grow for 2-3 weeks and then will be affected again.
I cannot determine what the cause of the growth anomaly is. Not all stems are affected at the same time suggesting that water conditions in the aquarium are not the cause.
I have seen references to similar problems by other aquarists growing this plant and one suggestion has been low phophate levels. While I cannot tell you what my phosphate levels are, this doesn't seem an adequate explanation as all stems should be affected simultaneously, if this was the case.
Any clues on what might be going on?