The tuner(s) issue is troubling...
I'm concerned that a hard drive upgrade might not be warranted at the moment.
However, yes save money and pickup the 250GB drive if you decide to go that route. You can only use 137GB of that 250... but save $10... and you'll still have a 250GB drive should you ever use that drive in another device...
So... I'd personally run down a list to eliminate "other" issues that may be causing a tuner problem. It might still be a bad tuner(s)... but it is best to be certain.
1. Power down the UTV overnight allow it to cool down. Make sure it is in the open, clean, and the internal fan is running. Elevate the base to increase air flow (bottle caps)... improve cooling if possible. If you are lucky this may extend the life of your UTV and reduce overheating related problems.
2. Pull the Tuner Cables from the UTV and see if the UTV remains running and doesn't restart "when it hits a certain part" of the drive.
3. Test out the individual Tuner cable runs. Bypass the multi-switch (if you have one). Check the LNB (Dish) and all the contacts (grounding block) between it and the UTV. Test the lines individually (not in pairs). Test it on one tuner, then the other... then checking the other cable.
4. Consider "restoring" the UTV to an "out of box" condition. This will delete EVERYTHING except the UTV OS... so be sure to save everything you don't want lost before hand. Use UTV Code 2355 2355 from the UTV Remote. (Check the UTV FAQ for more details) This can be a good move before trying a replacement drive. Most software "glitches" that a reset can't fix... can be resolved this way.
Best of Luck... Hopefully its just an overheat issue and some TLC and adjustments can improve things.