Problems with Barco Graphics 801

Hi!
I have a Barco Graphics 801 which has been stored away for a few years, but now I decided to give it another try to repair it.
From the beginning the projector was set to ceiling mount. I'm not sure whether it had any problems, and I think it wasn't used much. Possibly it hadn't been used for years when we got it. If I remember correctly the stand-by counter was something like 18000 hours, but probably it was not actually on most of the time. The tubes look really fresh.
After switching the projector from ceiling mount to table mount (by changing the 4 switches, while the power was completely off) the projector was working for about 2 hours, and then R138 (4.7 ohms high power resistor) on the H-shift board (the one most to the left if you are behind the projector) burnt, literally; we could see smoke.
R138 is for the red channel. There are two more resistors of the same type (R128 and R118) for blue and green.

After replacing R138 the projector was working again. I only let the projector run for a couple of minutes, and then I shut it off to check the resistors. R138 was burning hot, while R128 and R118 were cool.
<20>
I switched the projector to ceiling mount to see what would happen. When I started the projector scan fail was blinking (maybe around 2-3 blinks per second) for a few seconds during start-up, and there was some "scary" crackle sounds (the kind of sound you don't want to hear from electronics, because you think it's going to burn). After the initial seconds, the projector seemed to work fine, for about a minute. Then there was scan fail (and HTHD went out, as it should, to protect the tubes) and "scary" sounds. After a few seconds of constant scan fail I cut the power. To my surprise, R138 was quite cool this time. (R128 and R118 were also fine.)
<24>
I tried to start the projector, which was still set to ceiling mount. Scan fail was blinking and there was "scary" sounds during start-up, just like the previous day. After the start-up I got picture, but now and then there was a scan fail (and "scary" sound), and the picture went blank for a second or so each time. After about a minute I cut the power. The three resistors were cool.
I switched the projector back to table mount (which is what I will probably use, if I get it to work). Now everything seemed to work just fine. There were no scan fails at all, neither during start-up nor later, and no "scary" sounds. All three tubes were on (I activated adjust mode). I shut the projector off after about a minute. R138 was hot, while R128 and R118 were quite cool. I guess R138 would burn within a couple of hours, if it would even last that long, which is what happened a few years ago with the original R138.
Any ideas about what could be wrong?
To summarize, I think the most important points are:with ceiling mount R138 is cool, but with table mount R138 gets burning hot, while R128 and R118 are still cool.
with ceiling mount there are scan fails, and "scary" sounds.
with table mount, the projector appears to be working fine (except for R138). No scan fails. The LEDs for each voltage are lit, as they should be.
That R138 gets burning hot must be a symptom of something else, not that there would be something wrong with the resistor, but what could it be? And why does it only happen at table mount?
Which components are used / stressed differently at table and ceiling mount respectively?
Are there any components that are used only for table or ceiling mount respectively?
Maybe I should mention that I'm very familiar with electronics, but except for this BG801 I have no experience in repairing projectors. Replacing components, soldering and metering is no problem for me, though I would prefer to not meter while the projector is running, and I don't have a raiser-board(?).
The problem is that I'm not sure what I should look for. Because of R138 I assume that the problem is something H-related on the red channel. At the same time I don't want to rule out that something else could be wrong as well, since the projector was acting in different ways depending on whether it was set to table mount or ceiling mount.
I'm very grateful for any suggestions about how I should proceed.