A lot of us have long forgotten those days when we got spontaneous erections, erections that occur without the slightest provocation. Definitions include words like “random” or “unwanted”. Of course, some guys can’t imagine any erection as “unwanted”!
For example, I had one man write in with the following story:
“I am 54 years old and a few weeks ago I started having spontaneous erections after years of not having them. I would drive over a bumpy road or move or sit during the day and they would occur every so often. I then took omega 3 capsules, 1-gram each for three days, 5 the first day, 4 the second day and 5 the third day because I read on the internet that omega 3 would help the testicle sack to hang lower and I thought it might improve appearance. After taking the omega 3 the spontaneous erections have stopped. Do you know how I can get the spontaneous erections back?” 
First of all, the fact that this guy, who is one year older than me as of this writing, was getting spontaneous erections is a sign that, underneath it all, his basic plumbing is working well and he just needs to do a little diagnosis. Spontaneous erections are not particularly well-understood, but researchers have uncovered some of the basic factors involved:
1. Testosterone. One study showed that “during treatment, the hypogonadal men reported increases in several aspects of sexual activity, including sexual interest and the number of spontaneous erections.”  By treatment, the authors mean HRT, or testosterone replacment therapy of course. Nocturnal erections were also returned in the hypogonadal men.
2. Nitric Oxide. Cialis (tadalafil) boosts nitric oxide indirectly by essentially allowing existing nitric oxide to persist longer in your arteries. So, if you give a man Cialis, and it improves something, it is very likely because net nitric oxide levels were increased. One study found that giving men Cialis increased their spontaneous erections.  Other nitric oxide boosting therpaies would likely also work and I have many of them here in my Summary Page on Improving Erections .
3. Dopamine. This is going to sound strange but researchers got pretty excited when they found in both animals and humans that yawning sometimes induces spontaneous erections. The reason? It likely shows that dopamine is involved in erections.  This is also shows by certain medications such as L-Dopa and apomophine that work on dopamine levels or receptors. 
NOTE: Could the fish oil have really stopped his erections.?Well, I suppose if it was contaminated or highly oxidized. In addition, fish oil does have a fair amount of saturated fat and some men are sensitive to this. But, in general, fish oil is considered vasodilatory, i.e. relaxes the blood vessels and increase blood flow, so that is a surprising outcome.
2) Journal of Andrology, July-August 1992, 13(4):297-304, “A Long-term, Prospective Study of the Physiologic and Behavioral Effects of Hormone Replacement in Untreated Hypogonadal Men”
3) European Urology, 48(2):326-332, “Resumption of Spontaneous Erections in Selected Patients Affected by Erectile Dysfunction and Various Degrees of Carotid Wall Alteration: Role of Tadalafil”
4) Pharmacology Biochemistry and Behavior, Jan 1985, 22(1):31 35, “Association of spontaneous and dopaminergic-induced yawning and penile erections in the rat”
5) European Journal of Pharmacology, Aug 1984, 103(1-2):81-89, “The yawning-penile erection syndrome as a model for putative dopamine autoreceptor activity”
6) Progress in Neuro-Psychopharmacology and Biological Psychiatry, 1984, 8(4-6):695 699, Effect of apomorphine, a dopamine receptor agonist, on penile tumescence in normal subjects”