W123, Diesel Engine Repair

I am currently dealing with a very frustrating starting problem with my 300D. This is the symptom: I turn the key in the ignition and sometimes it will start and sometimes it won’t. If it doesn’t start I proceed to turn the key between the off position and the start position. Eventually it starts, sometimes after doing this once, sometimes after 10 attempts. At the beginning of this problem, I knew nothing about automotive electrical systems, so I thought it was the starter and replaced it with a remanufactured Autolite starter from Kragen/Checker Auto Parts (I know this was probably a bad idea). Well, the problem continued and since I already dropped off my (probably) good Bosch starter for a core refund, it’s too late to go back.

I have a 240D 1981 and I recently performed a valve adjustment. The adjustment seems to have gone as planned but when I began driving it I noticed decreased power due to the fact that the tranny was shifting too early on all gears now. I may have 1st of all done the adjustment poorly or may have messed up some vacuum lines to the tranmission shifting system which I don't understand too well. Anyone have any hints?

installing the aftermarket block heater in the lower radiator hose making sure the arrow is pointing up. I'm sorry but I cannot see the arrow. The heating element in the block heater, would it be pointing down when installed? Thank you

oil is leaking into my car from somewhere up under the dash/cluster, and drips down onto the brake pedal and my shoe, and while it is not a huge leak, it is a messy pain in the ass, and i would love to figure out what is causing it, and fix it. i have tried looking up behind the dash, and it seems like pretty much everthing is coated in oil, and so it is hard to get a sense of where the leak is coming from. someone had suggested a faulty oil pressure gauge. how difficult of a repair job is this? do i need to take the dash out to solve this problem?