Which electrolyte source makes a big difference. On a 300k last year I used Isostar tablets, which are slightly effervescent and only slightly sweet. By the time I reached 200k I had a howling dose of the knock: trembling, palpitations, nausea, gungy mouth, short temper, the lot.
The cause, as I found out when I got home, was that the Isostar tablets use aspartame as a sweetener. Aspartame fools your metabolism into reacting to sweetness by secreting insulin, and if there's no sugar there for it to work on it eventually yanks your blood sugar level through the floor. The knock also kills your appetite, making you feel as if eating would make you vomit.
I remembered all this, of course, at around km 500 of our weekend 600, when I made exactly the same error again. Force-fed my face with crystallized ginger and all was well.
Anyway, watch out for artificial sweeteners: they have no place in a sports drink.