I've encountered the same problem in the past...

1. You are using cheap programs OR...
2. You are using inferior media (DVDRs)

If you are serious about converting movies, you should invest in a program that is not free. There are two programs available for about $50 that work excellent...

Tsunami MPEG Encoder (TMPGEnc) Plus 2.5
Cinema Craft Encoder (CCE) Basic

You should be able to find trial versions of these products, although there are most likely limitations on encoding.

My recommendations for media are:

Verbatim
Hitachi-Maxell
Taiyo-Yuden
Sony

DVD+R media is more widely recognized by players than DVD-R.

I also support the above recommendation of burning at low speeds. I use 1-16X media and I burn at 4X.

If you follow these recommendations, I am confident that you will solve your problem. For more advice, I would suggest looking into some video forums, i.e. Afterdawn, Doom9, etc.

Good Luck