Depakote Reaction or Loss of Effectiveness?

My husband is 37 and was diagnosed with epilepsy 19 years ago.  He has been on Depakote for almost 10 years, along with a host of other medications.  Five years ago, he was finally weaned down to 500mg of Depakote daily and has remained seizure and side effect free.  His med levels remained at just therapeutic levels (as most of you know therapeutic is considered between 50-100) and since there had been no problems, no adjustments were made.  Two months ago he had a grand mal seizure, seemingly out of the blue (no warning petit mals that he had had in the past).  They upped his meds to 750 mg daily with almost no effect on his levels.  Another increase to 1000mg daily of Depakote and his med levels remained at about 60.  Then last week he had another grand mal, again out of the blue.  Blood levels were tested within 2 hours of the seizure and were at 62.  The neurologist decided he needed to be on the high end of therapeutic, so Depakote again was increased, to 1250mg daily and med levels have only gone up to 87.  As a result of these increases, he is suffering from short term memory loss and is fumbling for words.

Three weeks prior to the seizure 2 months ago, he began taking Zocor and Lopid, for cholesterol and triglycerides respectively and we had also adjusted to a lower carb diet.  These medicines were the only thing new that had been introduced in the last year. 

Has anyone had experience with these medicines (Zocor or Lopid) affecting the effectiveness of Depakote or any other seizure medicine?  Or can Depakote simply lose its effectiveness over time?

Any insight would be greatly appreciated.