Quartz Scheduler in Weblogic 8.1 SP3

[Dan]

I have a J2EE application running on Weblogic 8.1 SP3. A servlet is
responsible for scheduling jobs using a JobStoreTX on Sybase 12.5. All
references to Quartz are confined to the WebService layer of the
application. From time to time, just when the job is about to be
executed I get an

java.lang.NoClassDefFoundError: org/quartz/ValueSet
at org.quartz.CronTrigger.getValue(CronTrigger.java:1630)
at org.quartz.CronTrigger.storeExpressionVals(CronTrigger.java:1283)
at org.quartz.CronTrigger.buildExpression(CronTrigger.java:1119)
at org.quartz.CronTrigger.setCronExpression(CronTrigger.java:690)
at org.quartz.CronTrigger.<init>(CronTrigger.java:641)
at org.quartz.impl.jdbcjobstore.StdJDBCDelegate.selectTrigger(StdJDBCDelegate.java:2397)
at org.quartz.impl.jdbcjobstore.JobStoreSupport.retrieveTrigger(JobStoreSupport.java:1097)
at org.quartz.impl.jdbcjobstore.JobStoreSupport.acquireNextTrigger(JobStoreSupport.java:1724)
at org.quartz.impl.jdbcjobstore.JobStoreTX.acquireNextTrigger(JobStoreTX.java:1182)
at org.quartz.core.QuartzSchedulerThread.run(QuartzSchedulerThread.java:231)

It seems to be a run time ClassPath problem, although Quartz.jar is on
the path. Has anyone experineced similar problems?

Thanks,
Dan