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That's most likely your problem. Use a digital thermometer with a probe, or even better yet, one of those laser temperature guns.. Also, you want to measure the temp directly at the basking site, not the ambient temp up on the wall.
If you find that Tetra tank on-line, you will be shocked by the price, I guess that's due to shiping. Amazon had it for about $600. The local shop I got mine from was $235.
If you decide to build your own, just use 1/4" plate glass. (not tempered).
Actually, Tetra makes a 48x18x21 sliding front terrarium. The big chain pet stores don't carry them, but small shops can order them.
http://www.tetra-fish.com/Fauna/Products/reptile-terrarium-cages-enclosures/deluxe-reptohabitat-reptile-enclosure-48x18.aspx