There cud be two things and both are quite different.
If spark plug is due for change you can experience this type of issue or may be spark delivery was weak coz of weak igniton coil or faulty HT lead then also you may have this type of loss of power.
Second and most dangerous doubt is engine head gasket is leaking and coolant is getting mix with engine oil result in such loss of power behavior along with excessive white smoke and coolant turns milky.
See what you got, i hope its first one, Its less expensive (200 - 800) than the second one (4000 - 8000).