‘79 300SD Can Be Hard To Start…

I recently purchased a 300SD with a little over 300,000 miles on it. It runs really well when it’s warmed up, but even with the summer heat I’m having trouble starting it right up if I haven’t driven it in 6+ hours. It turns over for a while the first try and when it eventually starts, it bumps around before turning back off. It takes me about 3 tries before it starts, and then it’s a little bumpy for the first 15 seconds before I shift into drive. It’s also leaking some baseball-sized oil spots, though I understand that this is characteristic and was assured so by a few people. Apparently the glow plugs were replaced within the 6 months before I purchased it as well. Any clue as to what might be the issue?