Thanks, everyone, for your input! A lot to think about. I'll see if I can answer everyone's points.
First, it starts right up, every time. One shot of gas, and I'm good. On cold days (this is Arkansas), I use the choke, but still, starting is absolutely not a problem. It idles a bit rough, as if it needs a little more rpm. Idle speed is about 600 rpm. It doesn't die, though.
The rpm range where it isn't happy, for whatever reason, is 2100 to 2900. At 3000, everything smooths out. Trouble is getting to that point. Here in the Village, we have a lot of hills, both gentle and fairly steep. A typical scenario, but only since this problem started
, is that I'll be in 4th on level ground, running about 45 to 50 mph. Then the road will have a mild incline, and I have to drop down a gear just to maintain smooth running. Lots of engine noise, but no real power, and before this all started, I wouldn't have had to shift at all. Sometimes I have to drop down to 2nd, and that's ridiculous for the grade, but that's what it's taking to keep from chugging.
I think the suggestion to pull the fuel line and check is a good idea. Again, I feel strongly that this started after the last tank of gas, whether from bad gas from the station or running too low before filling up. That was why I put the Seafoam in, but I can't say there was any improvement. In fact, after cleaning and gapping the plugs the other day, I thought there was a little improvement, but driving it over to the doctor today (9 miles round trip), I'd say no. It ran like doo-doo, frankly, unless I was at 3000 rpm. Then all was well...until the road sloped upwards.
Back in the old days, when I flew, part of my preflight was to drain the sumps and look at a sample for water. Is there a way I can check the bottom of the tank without draining it? Or would I run into an obstruction if I tried to stick a tube down in the tank? (I don't see anything using a flashlight, BTW.) I've got some clear plastic tubing.
Compression check before purchase last fall was 160 on 2 cylinders and 170 on the other 2.
All 4 plugs were equally sooty, so no evidence of anything running lean at all.