After running for 20 to 30 minutes, engine looses power and stops. Will start up again immediately. Recently replaced spark plug, air filter, fuel filter and an oil change.

This type of issue has 3 possible senerios. In 35 years, I have experienced each of these by themselves, or sometimes multiples at the same time. This is a tough issue, but can be resolved with enough patience and a test area.

#1 Fuel Related
Depending on make and model of mowers:
a) Fuel tank outlet plugged (some have a screen filter, some are 90' angle and plug with rust and debris inside the corner of fitting)
b) Fuel lines collapse when they are routed thru fasteners and between frame and seat decks
c) Fuel cap vent clogged
d) Fuel filter clogged
e) any fitting that has an orifice that can trap debris or rust will eventually become clogged
f) fitting on the side of the carburetor
g) some systems have a filter inside the line at the opening of the carb inlet
h) fuel pump
i) vacuum hoses to fuel pump
j) debris under inlet needle/seat on carb

#2 Ignition related issues due to time and heat
a) Flywheel looses magnetism (not too common, but does happen)
b) Ignition switch or circuit breaker overheating
c) wire connection on plug-ins to switch or wire harness
d) points/condensor breakdown
e) ignition/armature coil
f) spark plug wire
g) electronic trigger/coil overheat

#3 Exhaust Valve and/or sticking due to overheating
a) valve stem is coated with excess carbon
b) valve face and seat are worn and do not seat properly after becoming warm
c) valve clearance too tight

I’m happy to assist further over the phone at https://www.6ya.com/expert/rolandw_c013176d62ee17ae