I sympathise; this also happens to me. It's true we don't always have the luxury of leaving a project when it no longer holds interest -- especially at work. And yes, the harsh but ideal solution is to find a job that supports an ever curious mind ... which is obviously extremely difficult to achieve in practice. You're not the first to express an affinity for monotone tasks (running, picking up litter, washing dishes) -- I think it lets us get into a zen or spaced-out state where we "just do". If only washing dishes were as lucrative as an engineering position!
Any chance you can apply some "agile" methodologies to your engineering role? My point is, if you work on a series of tasks with a narrow scope and an imminent deadline, that might be more overcomeable (yes, that's a word ... now) than having fewer, larger projects that seemingly drag out endlessly. You know, get the drop on your eventually dwindling motivation by being done before that even has time to kick in.
functional_tim