Are you using the HDMI output from the HTPC?
This sounds like an EDID or HDMI handshake issue. When you turn off the receiver and/or TV, the whole "smart"
HDMI chain gets a little screwed up. When you turn on again, I think the PC thinks the connected devices are much lower res (probably 640x480 or 800x600). MC may be getting this info at that point too. Then, when the PC resyncs, perhaps MC isn't doing so.
The most reliable way to fix this (if it is what I suspect) is to get an HDMI Detective from Gefen.
Here's an Amazon review:
I was hesitant to buy this part mostly because of the price. As it is, building my Blu-ray HD HTPC ended up costing me about 3 times what I had originally anticipated! Here's yet another $100+ dollars ... "will it work and will it be worth it?" I wondered. Bottom line, I'm very happy I purchased this item because it does exactly what it is supposed to do.
I was having problems with my HTPC setup. I run HDMI carrying audio and video from my HTPC's motherboard to a A/V Receiver. Then, another HDMI cord goes from the Receiver to my HDTV. Before installing the HDMI Detective, the problem would occur whenever (a) my HTPC would go to sleep/standby and then I would wake it up; or (b) I would turn off my Receiver or HDTV and then turn them back on. In these scenarios, once I woke up the PC or turned on the Receiver/TV I would no longer have a video or audio signal via my HDMI cords. One solution was to do a hard restart on the HTPC, which isn't horrible, but certainly is a pain in the butt. The other solution was to leave all three components running 24/7, which I quickly ruled out as an option since cumulatively they consume about 1000 watts and we all know electricity isn't free.
So, I purchased the HDMI Detective and installed it between my HTPC and the Receiver. Plugging in the Detective was easy and the initial programing took about 5 minutes. This is a very easy-to-use device.
With the HDMI Detective in place between my HTPC and Receiver, now everything works more like a traditional PC. I can put my HTPC to sleep and when I wake it up the Receiver instantly sees the HDMI connection and sends the subsequent signal on to the HDTV. Also, when I turn off and turn back on my Receiver or HDTV, everything picks up on the HDMI signal as if nothing were the matter.
In conclusion, the HDMI Detective is an expensive item that you likely didn't anticipate needing to buy for your setup. But since you're reading this review you must be having HDMI problems. The HDMI Detective is 100% worth the price. I did some hefty research and I could not find any other items on the market that do the same thing that the HDMI Detective does, so that makes it even more worth the price. It is compact, easy to use, and most importantly, works perfectly.
Stop stressing, buy the HDMI Detective. It will solve your HDMI HARDWARE problems. Solving SOFTWARE problems is another story for another time. 
In the case of the review, resuming from sleep resulted in no video or audio, rather than being at the wrong resolution, which I believe is your case.
It's $91 at Amazon. I believe Monoprice sells the equivalent called the DVI Doctor; supposedly using DVI to HDMI cables in and out of the DVI Doctor gives you the same thing, including audio from what I've read. The DVI Doctor is about half the price of the HDMI Detective.
Alternatively
Maybe one of the JRiver staff will look into why the application doesn't resume to the same resolution as the screen upon wake up?
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Topic: When starting HTPC after long time idling, MC is too large for screen (Read 2820 times)