Trouble with spindle bogging down under normal load

I'm new to the forum but I have been using all the great info found here for quite some time. I'm having trouble with a Kia skt 25 lathe. The spindle has started slowing significantly under a normal load such as say drilling a hole at 1.875 diameter. Checked the belts for tightness and the belts were sufficiently tight although quite dirty. Cleaned the belts with some improvement. It seems as though when the machine has been running a couple hours this problem slowly goes away. Does anyone have any suggestions on what i should look for to find the root cause of this issue? Our shop is still using spade drills by the way. Drill running at 400 rpm with .012 feed per rev in 1018. Drilled 1000s of holes on this lathe with no problem until this past week.

Does load meter rise or just slow as if something is ipping?