Well, if you randomly guess, you SHOULD be getting most questions wrong! You should only expect to guess the right answer about 20% of the time - i.e., one out of five times since there are five answer choices.

As for a timing strategy, never, never, ever leave questions blank. You have a 0% chance of getting a blank question right, as opposed to 20% for a random guess. Also, long strings of wrong answers hurt your score more than do the same number of wrong answers sprinkled throughout your test. So, your guessing strategy should be to pick and choose which problems you want to guess on - wait for a particularly hard problem (one that you'd likely get wrong, anyway), guess on it, and move on. That way, you don't have long strings of wrong answers and you're proactively choosing which problems to guess on rather than simply guessing on the last few - some of which you may have gotten right had you saved some time.

Hope this helps!