Which bulb "number" are you using? I've been thinking about trying the Silver's in the fog but AutoZone and Advanced don't have them. Store or on-line. Maybe not a good idea?????
Could be a heat problem not near enough volume/area in the enclosure as compared to the headlight enclosure.
Never seen any specs as to the temperature difference from a standard halogens. Not being a lighting "guru", I know the color temp is higher and this is the part that I've never studied, how the color temp equates to actual bulb temperature. I remember the when halogen filled bulb came out the filament temp was much hotter (compared to incandescent vacuumed enclosure)but the halogen re-deposits the self-destructing filaments molecules back to the filament. However never read anything about the actual physical temp.
That's the problem with Mods, change one thing and something will else will need changed. The enclosure was designed for (893)38 watt halogen. If the same bean counter design applies as compared to everything else on the vehicle not much room is left for the fudge factor!!
I have tried the ( 883) 50 watt bulb but it does not the cap on it to help with the cut-off just a brash light. I've yet to find anyone that made real fog that fits without mods to the area. Hella makes a projector (universal),but not very affordable. It appeared to have a very useable pattern.
Ah yes that brings up another point, 99% of all fog lights offered are decorations/marketing toys only or some very objectionable/blinding I.e.: certain Pontiac models. Say look at me "I have fog's and I'm something". Or I don't know how to shut them off!
I guess as long as the standard headlights meet SAE specs everything else added doesn't require any engineering ethics. i.e. marketing says it will sell more cars. Form over function.
Paul
I would zap them a note... 
