What sort of bulb are you using?
I was using a 60 watt ceramic bulb for my 20 gallon long tank, but my snake was telling me that it was too hot (pressed up against the glass on the other side or crammed in his water bowl), though the temps were well within range. I've since changed it to a 30 watt and he still seems to prefer the cool side, though he is now thermoregulating more.
I'd try a ceramic bulb to see if that will work. They are more expensive that a regular bulb, but they work better and last longer, IMO.
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alstiver@hotmail.com
Current snakes:
1.0 '01 Hypo snow cornsnake (Tesla Coil)
0.1 '02 Ghost (pastel) cornsnake (Banshee)
1.1 '02 Bloodred cornsnakes (Desi and Luci Too)
To be added in February:
0.1 '98 Het Hypo, Het Caramel cornsnake
1.0 '00 Hypo Het Caramel cornsnake