I am not sure "how" Spectrum's DVRs record shows, but is it possible that their is noise on the cable drop / channel that is being recorded, and the DVR aborts recording if the signal drops and becomes unreadable?
Does the behavior always occur on the same channel? Or is it completely random?
Did the recording get cut short due to a power outage or perhaps all the tuners being in use and something had to be cut?
This is the assessment that must be checked. Do you have a two -tuner box, a four-tuner box, or a six tuner box? The number of tuners indicate how many shows can be recorded at the same time. If too many tuners are being recorded, the latest recording will be dropped. If watching TV and you try to change channels, you will get a conflict message with on-screen steps that one show must be deleted.
However, my suspicions are that since this is happening twenty minutes into a sixty minute show, where forty minutes is lost, the next time this happens, make a note of the channels that are affected, and their recording times.
You may have signal or line problems and would need to set up a signal and drop line test. Interference known as noise or ingress is building up on the line affecting a channel or channels When you call Spectrum, have them put a signal and drop line test in the work order.
Important- Do not reboot any boxes, modems, or routers, for at least six hours before your tech visit, as this will reset the error codes that he will read off the channel back to zero. We need to see the errors.
When the tech comes out, tune to each channel that shows the twenty minutes into a hour long show, program loss of picture, Have the tech check your drop line, basement wiring, and main tap. Bad weather, animal chews, aging line drops and wiring can cause signal problems.
A swapped box will NEVER solve bad signals or line problems. Remove any splitters if you have them connected, they can diminish signal quality by up to ninety percent! Remember, no reboots of any Spectrum equipment for six hours before your service call.
If your signals and wiring are good, and the number of tuners and drop and tap must be checked by a tech to verify, than, and only than, would the box need to be replaced.