I've got a '87 300D with 247Kmi. At 77 MPH, the engine usually runs at 2950-3050 RPM, uphill, or flat ground. However, SOMETIMES, it will run 3400 RPM up the same hill that it usually will do at 2950-3050. The only thing I can think - I think it runs faster only when the car is hotter. Is this normal? Seems to have only started doing this more recently, but the weather is hotter than its been.

Also, I've frequently seen the car stay at say a specific RPM, maybe 2200 RPM, and the speed will increase, while the tach stays the same. Is this normal?

Thanks,
Brian

Most probably either your speedometer or tachometer is missimg a bit.
The other possibility is that your transmission maybe slipping a bit but I am sure you would have noticed that, so i doubt that this is the problem.

i believe it's a situation where the torque converter is locked up vs not locked up.

with the torque converter slipping (i.e., in non-locked-up mode), the slippage is converted to torque, hence the term torque converter. This happens at high load, and your engine RPM will be higher than usual for a given gear and vehicle speed. The less viscous the transmission oil (e.g. at higher temps), the more slippage you can expect.

As the load decreases, the slippage decreases, and the torque converter approaches lockup mode, and your speed increases with engine RPM constant, in the same gear, until the lockup point.