two-finger scrolling "jumps" unpredictably

When attempting 2-finger scrolling, it often happens that the document / webpage will scroll down by more than a full screen height at the instant I first lay fingers on the touchpad. This happens across multiple apps (text editors, web browsers).

Note: the system (Dell Inspiron 7380) came pre-installed with Windows; I erased the HD and installed Xubuntu 18.04 in its place.

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: xserver-xorg-input-synaptics 1.9.0-1ubuntu1
ProcVersionSignature: Ubuntu 4.15.0-52.56-generic 4.15.18
Uname: Linux 4.15.0-52-generic x86_64
ApportVersion: 2.20.9-0ubuntu7.6
Architecture: amd64
BootLog: Error: [Errno 13] Permission denied: '/var/log/boot.log'
CurrentDesktop: XFCE
Date: Wed Jun 26 14:23:35 2019
DistUpgraded: Fresh install
DistroCodename: bionic
DistroVariant: ubuntu
InstallationDate: Installed on 2018-12-23 (185 days ago)
InstallationMedia: Xubuntu 18.04.1 LTS "Bionic Beaver" - Release amd64 (20180725)
LightdmDisplayLog: (II) AIGLX: Suspending AIGLX clients for VT switch
Lsusb:
 Bus 002 Device 002: ID 706b:6b31
 Bus 002 Device 001: ID 1d6b:0003 Linux Foundation 3.0 root hub
 Bus 001 Device 003: ID 0c45:6715 Microdia
 Bus 001 Device 004: ID 8087:0025 Intel Corp.
 Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
MachineType: Dell Inc. Inspiron 7380
ProcKernelCmdLine: BOOT_IMAGE=/vmlinuz-4.15.0-52-generic root=/dev/mapper/xubuntu--vg-root ro quiet splash vt.handoff=1
SourcePackage: xserver-xorg-input-synaptics
UpgradeStatus: No upgrade log present (probably fresh install)
dmi.bios.date: 08/23/2018
dmi.bios.vendor: Dell Inc.
dmi.bios.version: 1.2.1
dmi.board.name: 0GW3KR
dmi.board.vendor: Dell Inc.
dmi.board.version: A00
dmi.chassis.type: 10
dmi.chassis.vendor: Dell Inc.
dmi.modalias: dmi:bvnDellInc.:bvr1.2.1:bd08/23/2018:svnDellInc.:pnInspiron7380:pvr:rvnDellInc.:rn0GW3KR:rvrA00:cvnDellInc.:ct10:cvr:
dmi.product.family: Inspiron
dmi.product.name: Inspiron 7380
dmi.sys.vendor: Dell Inc.
version.compiz: compiz N/A
version.libdrm2: libdrm2 2.4.95-1~18.04.1
version.libgl1-mesa-dri: libgl1-mesa-dri 18.2.8-0ubuntu0~18.04.2
version.libgl1-mesa-glx: libgl1-mesa-glx 18.2.8-0ubuntu0~18.04.2
version.xserver-xorg-core: xserver-xorg-core 2:1.19.6-1ubuntu4.2
version.xserver-xorg-input-evdev: xserver-xorg-input-evdev N/A
version.xserver-xorg-video-ati: xserver-xorg-video-ati 1:18.0.1-1
version.xserver-xorg-video-intel: xserver-xorg-video-intel 2:2.99.917+git20171229-1
version.xserver-xorg-video-nouveau: xserver-xorg-video-nouveau 1:1.0.15-2