PWDAD,
Since "arm pump" is a catch all term for various degrees of forearm fatigue ranging from weakening grip to severe "rigor" like cramping of the hand, the causes are likely variable. Electrolyte imbalance has long been implemented as the culprit and probably is for some of the cases. Depletion of immediate energy sources is also a likely candidate as well as the acidic byproducts of muscle contraction disabling release of ions needed for contraction.
So where do you fit in the puzzle:
If you get cramps or arm pump fairly quickly during a race and they do not go away than I would focus on training. This is to separate out the high anxiety "death grip" that we all have likely experienced early in a race that subsides on its own after we relax. The energy that supplies the muscle is actually needed to unattached the "bridges" that form to shorten muscle. Therefore, no energy available, the muscle is kind of "locked" into a contraction (rigor mortis like) although you are hopefully still living. Wrist curls, wrist rolls (my favorite), squeeze balls or grips, throw away wrist wraps if you use them to lift and of course, lots of saddle time pushing into arm pump. Training will in effect increase the availability and efficient use of energy sources, thus delaying the onset of cramping and fatigue.
Electrolyte imbalance will usually rear its head later in a race especially in an enduro or harescramble, although if you are imbalanced before the race it could come earlier. I can not stress enough the importance of prehydrating and rehydrating with a sports drink or supplement that has the basic electrolytes: Potassium, magnesium, sodium etc.. If you preload days before a race with strictly water and likewise on race day you are actually leaching electrolytes out of your body (remember osmosis). If you favor water, get the electrolytes through food intake. This will set you up for early onset of fatigue and if it is hot and you sweat a lot it could be much worse since additional electrolytes are lost in sweat. Low sodium with heat and heavy exertion can be deadly, I have seen this first hand at marathons. Although not an absolute, muscle cramping from electrolyte imbalance usually feels like a failing grip and not as much locking of the hand.
Good old fashion muscle fatigue is usually caused by a decrease in the availability of calcium needed for a contraction. This is not a systemic problem as all the calcium supplement in the world wouldn't help. It is there it just can't get where it needs to be because of the byproducts of contraction impede its release. This can also be impacted by training.
By know means are these phenomena mutually exclusive and could easily occur together. There really is no substitute for additional training and riding as they are the key and will impact on the causes as you will become more efficient in storing and extracting energy from the muscle as well as buffering byproducts and utilizing electrolytes.
BTW, this topic has been debated by exercise physiologists for years and there still is no consensus, this includes my office :) This is likely due to the multiple factors at work and dumb co-workers :D, nothing is simple! I don't mean to knock on anyones ideas and I say if they work, than go for it.
We didn't even mentione ergos on the bike: bar-setup, grips, stabilizer, rider positon etc..
Good Luck, Keith