Last winter I suffered with a truck that didn't have heat. The 2 trucks I drove in the winter prior to this one were the same way, I figured that's how they were being diesels.
So I blocked my radiator with cardboard to try to warm it up. Didn't do any good.
Turns out it was the T-stat. This spring I got one from Autozone, it lasted 3 days. Replaced with NAPA, it lasted 2 months. Replaced that one with 203 from Dieselsite, and it's been 4 months now I think.
One of the best things with the 203 is that the other stats stay open till the water drops down to 175* or 180* or something. Bob's 203* closes at 195 I think, so it keeps the heat longer.
It looks exactly like my NAPA stat, down to the exact markings etc, which I was concerned with getting ripped off. But it performs so much better than the NAPA stat did, it's got to be different. I could tell by the temps on the dash gauge.
Go for it. Get a new water neck, too. You'll kick yourself if you don't. The first 2 stats I dealt with the used neck, then when I got Bob's stat I got a neck, too. Worth every penny.