An important feature in Real-time systems is {\em parameter impreciseness} i.e. the inability to accurately determine certain parameter values. The most common such parameter is {\em task execution time}. A second feature is the presence of complex relationships between tasks that constrain their execution. Traditional models do not accomodate either feature completely: (a) Variable execution times are modeled through a fixed value ( {\em worst-case} ), and (b) Relationships are limited to those that can be represented by precedence graphs. We present a task model that effectively captures {\em variable task execution time}, while simultaneously permitting arbitrary linear relationships between tasks. Our model finds applications in diverse areas such as real-time task scheduling, compiler scheduling, real-time database scheduling and machine control. This paper focuses primarily on the computational complexity of answering queries posed in our model; in particular we demonstrate the existence of constraint classes that make the scheduling problem {\em hard.} (Also cross-referenced as UMIACS-TR-2000-16)