Relax. I know it is easier said than done. If there is tension in your arm and you are trying to play long slow bows, you are fighting a very uphill battle. Stretching helps, and I would suggest taking a break when practicing, especially if you are noticing tension. Even a couple of seconds where you consciously stop, set down your bow, and shake out your arm helps. You can even do this in a rehearsal as long as you don't flail too much and catch the conductor's attention.
Too much rosin can be a problem as well. If you are truly relaxed and playing long slow bows, your bow can get stuck in a particularly rosin-intensive spot. This might stop the bow all together, but usually the "solution" is to dig in so you can power through it. Try removing any build up from your strings and using a clean dry cloth, wipe some of the rosin off of your bow hair. You will need to use more natural weight in your arm to achieve loud dynamics instead of letting the rosin do the work for you, but it can really smooth things out.
Finally, experiment with where your bow is between the fingerboard and the bridge. Usually it feels "easier" up close to the fingerboard, and has more bite and is "harder" to play legato by the bridge. With a relaxed arm, you can get the feeling of being closer to the fingerboard when closer to the bridge, and that feeling of increased resistance allows you to slow down your bowing arm for longer bows.
I haven't mentioned your wrist yet, because I am guessing it is more of an arm problem. If your wrist is relaxed, you are using less rosin, and allowing the weight of your arm play the bass, I think that should make things easier. Especially with slow tempos, your wrist should be very quiet. Everything your bow does should be coming from your back muscles, and your arm/elbow/wrist/fingers all just relax and go with the flow.
Personally, I would be working on scales and long tones instead of exercises, but whatever works for you. Set your metronome slow, and try to get 8 beats out of a single bow. Start with two whole notes tied together, and then you can move to slurred whole notes, half notes, quarter notes and eighth notes (which is two octaves in one bow) all at the same metronome marking. You should be using the same bow distribution because you are still playing 8 beats in one bow, you are just moving your left hand. When you can do that comfortably, turn the metronome down a little slower. Repeat. When your metronome can't go any slower, start at your initial tempo and do the same with 16 beats to a bow. When your metronome can't go any slower, go to 24 beats. If you can do that and still be getting an even tone, Audition for Concertgebouw or the NYPhil or something. Legend has it Tom Monohan use to teach hour long lessons this way, where you would play a 3 octave F major scale, and be out of time at the end of it.
