Hello,
I have a 48in extra by extreme flight and a value hobby slick foamy. When i roll both at a moderate speed they roll flawlessly. when i slow them down and put them up for a harrier roll they complete about 3 or 4 rolls fine and then the plane will kinda slowly roll and then snap back into the regular roll speed. im not sure what the problem is but i just crashed my foamy due to this and i would like to know what the issue is because none of my larger planes besides my 50cc do this. i would like to find out why before lossing another plane. Thank you

Lots and lots of variables, almost too many to really pinpoint. Generally, rolling to the left will help because you're not fighting the torque of the prop. Just need to keep holding inputs and delay them while the airplane starts to roll slower. Also vary your throttle inputs to help hold the airplanes attitude up.

A good learning video from Mark Leseberg that could help.

Mark Leisberg Harrier Rollin (4 min 40 sec)

Angle of attack has the biggest effect on Roll rate especially if you roll right. Rolling right as the plane goes high alpha you fight the motor torque, it's not really a problem till after 45 degrees. As the plane gets closer to vertical the motor torque will stop the roll completely. Rolling left you can rolling harrier right into a torque roll. I'd say your letting the nose get too high.