First thing I'd look at is overtraining. Any time the times start to go backward, i'd take a hard look.

Past that, another thing to consider is how long ou've been doidna certain type of workout. i was just reading a paper showing that after 6 to 8 weeks you'll stop improving in swimming by adding speed to a given number of repeats.

On the other hand, repeats to a given set at the same speed will work for a bout 16 weeks. If you're at the edge of utility of one of these types fo workouts it might be time to cinsider shaking things up a bit.

I would also make sure that you are really focusing on stroke. Sometimes when you start training a lot in the water you lose focus on the stroke and just focus on completing the set. This has more of an affect than you would think. Also, have you made any adjustments to your stroke? Sometimes adjustments can really set you back for a while.

In response to your question on stroke. Mainly crawl is okay as long as you are doing enough of the other strokes that you are comfortable in them. Doing other strokes will help to keep you body balanced and prevent injuries but the other stroke stuff only needs to be about 10 - 15% of your workouts.

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"which is like watching one of your buddies announce that he's quitting booze and cigarettes, switching to a Vegan diet and training for triathalons ... but he's going to keep snorting heroin." Bill Simmons, ESPN