Auto-scroller in QScrollArea?

I have created an AutoScroller inside my Gallery but there seems to be a bug that keeps accelerating the speed of scrolling which makes the speed spinbox so unaccurate.

My Idea of autoscrolling is using the QTimer

from PyQt5.QtCore import *
from PyQt5.QtWidgets import *
from PyQt5.QtGui import *

class Main(object):
    def setupUI(self, Project):

Project = QWidget() Assigned from the outer scope and referenced

        Project.setFixedSize(QSize(900, 400))
        self.mainScroll = QScrollArea(Project)
        self.mainScroll.setGeometry(QRect(80, 0, 740, 400))
        self.mainScrollWidget = QWidget()
        self.layout = QVBoxLayout(self.mainScrollWidget)
        self.layout.addStretch()
        self.layout.setGeometry(
            QRect(0, 0, self.mainScroll.width(), self.mainScroll.height()))
        self.mainScroll.setWidget(self.mainScrollWidget)

        self.checkBox = QCheckBox("Auto-Scroll", Project)
        self.checkBox.setGeometry(QRect(0, 0, 80, 30))
        self.checkBox.toggled.connect(lambda: self.StartTimer(Project))

        self.PX_PER_TIME = 1
        self.Speed = 10

        for i in range(200):
            MainWidget = QWidget()
            label = QLabel("LABEL NO_%s" % str(i), MainWidget)
            label.adjustSize()
            self.layout.setSizeConstraint(QLayout.SetFixedSize)
            self.layout.addStretch(1)
            self.layout.addWidget(MainWidget)

    def ScrollDown(self):
        vScrollBar = self.mainScroll.verticalScrollBar()
        vScrollBar.setValue(vScrollBar.value() + self.PX_PER_TIME)

    def StartTimer(self, Project):
        if self.checkBox.isChecked():
            self.timeleft = self.Speed
            qTimer = QTimer(Project)
            qTimer.timeout.connect(self.TimerTimout)
            qTimer.start(1)
            self.ScrollDown()
        else:
            if type(qTimer) != None:
                qTimer.stop()
                self.timeleft = self.Speed

    def TimerTimout(self):
        if self.timeleft > 0:
            self.timeleft -= 1
        if self.timeleft == 0:
            self.timeleft = self.Speed
            self.ScrollDown()

Edit

This should reproduce my problem and it kinda explains the type of solution, What I am trying to accomplish is completely stopping the QTimer once the QCheckBox is disabled and reassign all variables to their base state and stop the scrollbar from scrolling.

here goes my answer. I've implemented a simple example built from the top of your question example. I fixed some syntax error and removed some unnecessary code. I hope it helps you out.

from PyQt5 import QtCore, QtGui, QtWidgets


class Main(QtWidgets.QWidget):
    def __init__(self):
        super(Main, self).__init__()

        self.speed = QtWidgets.QSpinBox(self)
        self.speed.setValue(10)

        self.autoscroll = QtWidgets.QCheckBox(self)

        self.imageLabel = QtWidgets.QLabel()
        self.image = QtGui.QImage("test.jpg")
        self.imageLabel.setPixmap(QtGui.QPixmap.fromImage(self.image))

        self.ScrollArea = QtWidgets.QScrollArea()
        self.ScrollArea.setBackgroundRole(QtGui.QPalette.Dark)
        self.ScrollArea.setWidget(self.imageLabel)

        self.autoscroll.toggled.connect(self.toggleTimer)
        self.layout = QtWidgets.QVBoxLayout()
        self.layout.addWidget(self.speed)
        self.layout.addWidget(self.autoscroll)
        self.layout.addWidget(self.ScrollArea)
        self.setLayout(self.layout)

        self.qTimer = QtCore.QTimer(self)
        self.qTimer.timeout.connect(self.timerTimout)

    def toggleTimer(self):
        if self.qTimer.isActive():
            self.stopTimer()
        else:
            self.startTimer()

    def stopTimer(self):
        self.qTimer.stop()

    def startTimer(self):
        self.qTimer.start(1)

    def timerTimout(self):
        self.scrollDown()

    def scrollDown(self):
        scrollBar = self.ScrollArea.verticalScrollBar()
        scrollBar.setValue(scrollBar.value() + self.speed.value())


if __name__ == "__main__":
    import sys

    app = QtWidgets.QApplication(sys.argv)
    widget = Main()
    widget.show()
    sys.exit(app.exec_())

P.S. I've implemented an option to stop scrolling. :)