I don't know how old your are, but this is like a middle agers' problem. I have associated this problem with an insufficient charge of the parasymapthetic nervous system. Erection is supported by this nervous system. When the penis erects, the brain draws all the bioelectricity (Chinese call it "Chi") from other organs to support sex. When the brain detects the bioelectrity is too low in other organs, the brain will withdraw its support for erection and trigger the adrenal medulla to secret the stress hormone (adrenaline)to turn on the sympathetic division for "Flight" - runaway.
To prevent this from happening, you have to recharge your Automatic Nervous System (ANS). Since you are starting to experiement this problem, I suggest you to take Real DHEA and PeniSOS (with Stamina tea) to recharge your system. To produce a special effect for an occassion, you can take 1 tab of PenISOS, and drink 30 drops of Stamina and Ginseng Power Max 4x, 1-2 hours prior to sex.
Stamina and Ginseng will promote your endocrine, blood-circulation and nervous systems for sex at the same time. These products belong to LastLonger in
http://www.healthtao.com/herbs/last.htm
which includes Kava Kava.
However, it requires a sufficient level of DHEA in your bloodstream. Since your adrenal cortexes may not be able to secret enough DHEA, you need DHEA supplement. Real DHEA, including DHEA and the prostate-protecting herbs, can do the best job for your system. So, you need Real DHEA to boost your nervous system for 2-3 months. After your system goes back, you can take Real DHEA once a while.
You don't need Kava Kava.
If I can replace Kava Kava by Real DHEA for you, I will give a new name, called Endura from the word "endurance", for this combination. In the past, we advised seniors and middle agers take Rejuvenator.
If you are older than 50, you can try our Rejuvenator which include a testosterone booster called CyclE 4x.
You also have to change your diet too. Don't let your blood vessels get clogged by red meat and dairy products. You should double your vegetable and fruit intake and reduce your meat intake by 50%. Sea foods are good for you too. You should thin your blood. Ginger, Garlic, Parsley, Capsium, Golden Seal Root and Eleutherococcus senticosus can equalize your blood pressure and thin your blood. A good blood circulation in the genital area can help you retain your erection. And, you heart does not have to work too hard to irritate your symapthetic nervous system.

You need DHEA with the herbs that can protect your prostate. That is why I select Real DHEA. The Herbal components are very critical for the DHEA users.
Real DHEA will componsate the weak function of your adrenal cortex, the source of "Kidney Yin Chi".
You need zinc and soy, which are included in PeniSOS, to stablize you testosterone in the bloodstream and protect your prostate and liver.
You should directly add some testosterone building substances, which are in PeniSOS too, to support your testosterone burst when you engages in sex.
You should enhance your blood circulation, endorine and nervous systems at the same time to prevent the brain from tirggering the secretion of the stress hormone from the adrenal medulla. If this happens, your sympathetic nervous system will disable your erection.
Which products can help you out?
A combination of PeniSOS, Stamina and Ginseng - the Endura package.
Stamina includes the sex stimulants - Yohimbe and Horny Goat Weeds, and the blood promoter - Dong-quai. Ginseng enhances the efftiveness of these products.
B12, C, E and Folic Acid can help your liver and heart to withstand the stimulation of this package, but cannot directly boost your body for sex.
We have tested the Endura package without including Ginseng.