What kind of timer do you use?
I have a feeling your density variations aren't related to the exposure time and that they just coincided with your print development process that's not in control yet.
I suggested it before, but I really recommend giving tray development a try, at least for one session. If you have space to exposure a print, you also have space to set up 3 small trays for small test prints. Work at room temperature so you don't have to worry about temperature fluctuations (small fluctuations don't matter anyway, especially at low temperature and long processing times).
Make a few identical test strips and develop them at room temperature for 90 seconds, 120 seconds, 150 seconds, 180 seconds. Ensure that the exposure is such that you have a black patch on each strip. If not, do it again and adjust exposure so that you have black somewhere. Place the strips that are developed at different times next to each other. The first strip that shows a black that's pure black and not blue, is approximately your minimum development time for room temperature. You'll notice that the strips processed at a longer time show very little difference; they may have slightly higher density or contrast, but the difference is slight.
Use the minimum development time to process a couple of test prints. See if the consistency problems go away. Once you get consistent results (across let's say 5 prints or so, at least) with your 10-11 second exposures, do the same with 3-4 second exposures. At this point you'll be better able to determine where the problem in your process is.
Sometimes it takes some effort dialing in your process; once you've got this down, it's pretty easy/straightforward.