I agree with the other replies: you're seeing the big difference between HDTV cable box and SDTV recording on the Magnavox. If it is already bugging you to this to degree, my honest advice is to return the Magnavox to WalMart, suck up, and pay Time Warner the monthly surcharge for the decoder box with built-in HDTV recorder. You simply will not ever be happy with how the Magnavox records: you're spoiled rotten by the pristine quality of the HDTV cable signal, but unfortunately no DVD/HDD recorder can capture that signal: they only record in SDTV, further trashed by the decoder box.
This is an important but easily overlooked point: there is no "free lunch" or bargain in HD-quality cable TV recording. People think the Magnavox is some kind of dirt-cheap miracle cure that lets you avoid paying the monthly charge for a TiVO or cable PVR: that is just plain wrong. All DVD recorders are limited to non-HDTV quality and they suck much worse with cable than they do with off-air broadcasts. If you want HDTV recording quality, you will
pay, and it will be a lot more than you want to pay. Reconcile yourself to this fact, or forget about recording altogether and just watch "live" cable.
The Magnavox can make stunning near-HDTV-quality recordings from its built-in off-air 16:9 tuner, but if you have cable instead of an outdoor antenna all bets are off. Cable forces you to use a decoder box and the line inputs of the Magnavox instead of its tuner, limiting recording quality to whatever borderline crap SD quality your cable company delivers thru the non-HDMI output of its decoder boxes. I have the same TWC Samsung box you have, cjells, and its standard-def output is crap. Since that output is all the Magnavox is capable of recording, thats all you will ever see when you play the recordings back. Even if you connect the Magnavox to a second HDMI input on your TV, it doesn't improve much- its still crap, because the decoder box output is much lower quality than what the Magnavox tuner can pull in from off-air broadcasts.
The only possible workaround involves spending another $200 to get a very high quality accessory conversion box that will take the analog triple-wire component RGB widescreen 16:9 signal from your cable box and translate it to a single-wire 16:9 signal the Magnavox would be capable of accepting. This bypasses the crummy standard letterboxed output of the cable box, feeding the Magnavox a signal thats closer to what its tuner can achieve with off-air-broadcasts. The question is, do you really want to sink $400 into this project, when it will net you only a slight improvement? It still won't be "true HDTV," and recordings will still look "grainy" compared to the direct HDMI connection between cable box and TV. Its really only worth going to extremes if you're a hard-core collector who makes a lot of DVDs.
If you don't actually have any interest in making DVDs, and just thought you would save a ton of money using the Magnavox as a PVR instead of paying TWC fees, accept defeat gracefully. Get your money back on the Magnavox and sign up for the TWC PVR. The cablecos have us all by the short hairs when it comes to HDTV, my friend.