Performance of Triggered Stored Procedures

Hello all... We are having erratic performance problems with triggers. There are times when an UPDATE to a table is quick, and the next day the same update might take in excess of 30 seconds. It appears that the triggered SP is doing a sequential scan of the host table and ignoring indexes. While trying to find the problem a 'return' was placed as the first statement of the triggered procedure. Performance is still unbelievably slow. The triggered procedure is fairly large +- 150 lines - could this have any effect? 1. Any advice will be appreciated. 2. Are there any debugging utils or techniques which we can use? We have tried using 'set explain on'. 3. What else can we investigate? We use Online v5.1. Any help would be appreciated. TIA Harvey