26W lamp ballast(?) troubles

[ICSK]

Hello, I'm having trouble with my 26W lamp. Both bulbs will work fine at home but when I take it out in the field that's where the problems start. It'll light them both the first cycle but subsequent on/off cycles result in only one bulb lighting and the other flickering. I'm told that means that there isn't enough filament heat?

Possible solution 1?
Now, before I explain anymore I know the easy answer. My current setup uses a modified sine wave inverter and battery. I'm sure if I switched to pure sine wave it'd work fine. There are two problems though. Cost and size. The only one that I've found that takes up a small footprint is the Samlex 120W, which has mixed reviews on Amazon and is a bit expensive at $100. If anyone can recommend one that takes up a similar footprint or vouch for the reliability of the Samlex it'd be much appreciated.

Possible solution 2?
Here's a photo of the circuit. I marked the portion that I believe lights up the bulbs. What I want to know is if I can just increase the capacitance in C4/5 so that both bulbs will light up despite using a modified sine wave inverter. If so, by how much? They're currently .1uF 400V.

http://www.amazon.com/gp/product/B004FELHUO/ref=oh_aui_detailpage_o02_s00?ie=UTF8&psc=1

Here are the 2 bulbs I'm using. LEDs are not an option unfortunately. The technology isn't there for UVC. http://www.lighting.philips.com/main/prof/lamps/uv/germicidal/tuv-pl-s/927902804007_EU/product

If there's something else I'm not considering please chime in.
Thanks for the help!

[Seekonk]

What photo of the circuit?  Is this for septic sanitation?  I would suggest a 120 to 15-24V transformer used in boost mode on the inverter.  That will boost the peak line voltage to a normal 160 instead of the 140 found in most inverters.

[Kjelt]

when I take it out in the field that's where the problems start

What is the temperature "out in the field" ? I am thinking that this might be the culprit, CFL's are notorious when it is cold (<5oC).
When they are on for a few minutes they will stay on usually so if you are powering it from a carbattery you might try igniting them in the warm car, let them come up to temperature and then take them outside.
For cold temperature there are special CFL's with a capacitive lint going right aside the tube, they are AFAIK not available in this particular version however 
Indeed the best option in cold environment is to buy a LED based lamp on 12 or 24VDC directly from the car battery.

[ICSK]

What photo of the circuit?  Is this for septic sanitation?  I would suggest a 120 to 15-24V transformer used in boost mode on the inverter.  That will boost the peak line voltage to a normal 160 instead of the 140 found in most inverters.

Circuit photos
No, this is for field collecting of fluorescent minerals. That's why I wanted emphasis on portability. Unfortunately I won't have access to AC power as well. My lamp is just a standard worklight with UV bulbs put in. Then it is powered by a 13.2V LiFePo battery + inverter.

Here's some eye candy to illustrate what happens when you put things under UV light

What is the temperature "out in the field" ?

Indeed that's a possibility I considered and I do go regularly collecting at night in temperatures as low as -1C. However, I'm able to recreate this effect at home, leading me to believe that it's a problem on the electric side.

Indeed the best option in cold environment is to buy a LED based lamp on 12 or 24VDC directly from the car battery.

I wish!!! If 250-260nm LEDs were viable I'd have made one. I hate working with bulbs! Last I checked (October? November?) it was $250 USD for a 10 mw LED from CrystalIS, the current leader in the industry.

[Seekonk]

Looked at the picture.  Does the AC go into a full wave bridge rectifier?  couldn't see from the picture.  I bought a number of defective inverters and I just ran CFL lamps and DVD players directly from 140V DC when the H bridge was shot.  A little more efficient.  If comfortable with the electronics, I would just increase the  B+ sense resistor value so the HV to the H bridge increases 10-15V.  That would be the lightest solution.  I see two caps on the lamp board, is that a voltage doubler (only two rectifier diodes)?  That would restrict you to keeping it AC.

[ICSK]

Ah how I wish the problem was that it was just a defective inverter... I've gone through a half dozen of these from 4 different manufacturers.

Here's another set of images. The rectifiers are shown in Q1/2. Unfortunately I'm new to this and can not tell if they are full wave.
http://imgur.com/a/NZIlg

I also do now know what a B+ sense resistor is. I took a look on Mouser and none of the parts look like these:
http://www.mouser.com/Passive-Components/Resistors/Current-Sense-Resistors/_/N-7fjcf