Three different dilute solutions of high molecular weight polymers in viscous, binary solvents were used in experiments performed in a cone-and-plate rheometer. The solutions all fall into the class of fluids referred to as "Boger fluids" and were previously used in studies of viscoelastic Taylor-Couette instabilities. Under prolonged shearing in the cone-and-plate geometry, these fluids all exhibited a decrease of the first normal stress growth function N-1(+)(t) from an initial plateau value to a second, lower plateau value. This behavior has been previously observed, but is here reported for widely used polyisobutylene-based Boger fluids for the first time. As in earlier studies (Magda JJ, Lee C-S, Muller SJ, Larson RG (1993) Macromolecules 26:1696-1706; MacDonald M, Muller SJ (1997) J Rheol Acta 36:97-109), the time at which this decrease occurs (the decay time) is much longer than the polymer molecule's relaxation time. Here, we focus on three issues: 1) the time-temperature superposition of the first normal stress growth function N-1(+)(t), including the decay time and the value of the second plateau, 2) the sample recovery time required to reproduce the initial plateau value of N-1(+) and the decay time, and 3) the relationship between the time scales for this decay of normal stresses and the onset of viscous heating induced instabilities in the Taylor-Couette geometry. Our results suggest that shear-induced conformational changes, possibly coupled to viscous heating of the sample, may be responsible for the decrease in the first normal stress growth function during prolonged shearing.