We live in Philadelphia and moved to a new place in August.
In a recent chilly day, I turned on the heating for the first time, and noticed the furnace was turning on and off every 10 minutes or so, but the blower continued to run. Our thermostat (an EcoBee3) was also losing power every time the heating turned off.

The furnace is a Bryant Plus 80, model #311AAV042090 (88,000 BTU/h 80% AFUE).
The error code displayed is code 33 (I copied the description at the end of this post).
The docs for it can be found here: http://www.catalog.bryant.com/wcs/dynamiclit_result/1,2759,CLI1_DIV42_ETI7480_PRD797_SIT70,00.html

Our regular HVAC technician came over, checked the air filter, that the vent is not restricted and that the draft is OK and that the exchanger coil is clean.
They replaced the limit switch with a new one, but the problem continued.

I then called a Bryant certified technician. On top of checking the same things as the previous technician, he also performed a temperature rise test which showed a return temp of approx. 84F and supply of temp of approx. 135F. (delta T of 51F).

This is according to spec, but the "puzzle" is that the house at the time was at 71F (and outside temperature was in the 50s) and it is unclear to me (and him) how come the return air arrives at 85F. The ducts seem fine as far as we can tell. We measured the temp at one of the return vents, and it was 71F.

He suggested replacing the limit switch again with another one with a slightly higher limit, and also cleaning the blower motor which he said seemed dirty. Beyond that he said the system may be oversized for our house.

Both technicians from both companies said that they have never encountered such a problem.

The house is approx. 1500 Sq. ft., 3 stories high, built in 1988. The furnace is from 2003.

I'm inclined to just replace the furnace, but before doing so, I am wondering how to figure out if the furnace is the issue, or the ducts...

Any ideas and suggestions will be appreciated, on both solving this puzzle, or in case of replacement, what is the proper size for our house.

Below is the info about the error code from the Bryant manual.
----
33 LIMIT CIRCUIT FAULT - Indicates a limit, draft safeguard, flame rollout, or blocked vent
switch (if used) is open. Blower will run for 4 minutes or until open switch remakes
whichever is longer. If open longer than 3 minutes, code changes to lockout #13.
If open less than 3 minutes status code #33 continues to flash until blower shuts off.
Flame rollout switch and BVSS require manual reset. Check for: - Restricted vent
- Proper vent sizing - Loose blower wheel. - Excessive wind
- Dirty filter or restricted duct system.
- Defective blower motor or capacitor. - Defective switch or connections.
- Inadequate combustion air supply (Flame Roll-out Switch open).