The PRE's stat is useful if you are trying find out if the timer is slow or too fast. When the script is only in normal mode and it's getting PRE's then you are probably running another script at same time. Try to use the second timer or third timer. You can change the timers for both scripts to find a good balance. Also be sure you really need the scripts running because if you have a full queue and working on a good batch do you really need a script to scrape for more at this time? It's always best to use the tools efficiently to make you work better and make more money.
You can click on the PRE stats and it will display other stats. It will display the total number of PRE's, PRE's received in ham mode, PRE's received in normal mode and error stats. The error stats is just for weird pages the script might receive but usually this should be 0. It's expected to get PRE's in ham mode because it is sending requests faster than mturk wants. Anything below 980ms will cause PRE's. I have broken up the stats into PRE's that it got from ham mode and from normal mode so you can figure out if the normal timers are too fast or if it's just the ham mode that is causing so many PRE's. Use this info to change things or maybe you need it very fast to get hits from throttled batches. PRE's may not be terrible so don't worry about it. Just be sure you know where they are coming from.